As a UK IMC rated pilot, I am permitted to fly IMC in all airspace other than Class A. In Scotland, there are quite a number of class D and F airways, and the CTRs and TMAs all all class D.
Would I legally be able to file and fly an airways flight plan from say Prestwick to Sumburgh?
EGPK TRN DCT GOW N560D KISTA EGPB
I can't see any reason why not, although whether this is a good idea is of course another matter entirely.
I could of course quite legally and safely file and fly the same flightplan outside eurocontrol, negotiating clearances as I go.
Yes...and if you file "I" you will be impressed at the better treatment you receive!
Although I'm not sure why you would go via TRN and there is no need to include KISTA
Yes this is 100% legal.
"Airways" is really a wrong term. There is no such thing. What matters, in license privileges, is the actual airspace class. The IMCR is no good for IFR in Class A.
The only gotcha with flying long enroute sections in Class D is that ATC are not aware of your no-Class-A restriction, they have no procedure for supporting that kind of flight (i.e. keeping you in Class D) and could just tell you to turn right/left/climb/etc into Class A.
This can happen because in "Eurocontrol IFR" the controlled airspace class is practically irrelevant.
This has actually happened, in the southern UK, and resulted in at least one "interesting" flight that I recall reading about.
I suspect that procedures were changed after that. While every "I" flight plan goes to Eurocontrol, and if it passes their computer validation they will distribute it to the enroute units regardless of whether it is in CAS etc (I have this from them in writing) you can get a situation where filing a low level route (say 5000ft) will result in the flight plan being chucked out at the UK ATC end and when you come to depart you get asked to contact London Info instead i.e. your IFR clearance has just been quietly trashed. I suspect one reason they (London Control mostly) chuck them out is to stop IMCR holders being handled by London Control and potentially being asked to enter CAS which will usually be Class A.
I have tried to find out the conditions that cause UK ATC to discard validated IFR flight plans but nobody would reveal it.
There seems to be an exception for IR test flights (EXAMxx callsign) which are allowed to pop up into Class A for the obligatory 10 minutes 
Scotland is more viable in this respect because there are long bits of Class D, but you still need to watch it...
The only gotcha with flying long enroute sections in Class D is that ATC are not aware of your no-Class-A restriction, they have no procedure for supporting that kind of flight (i.e. keeping you in Class D) and could just tell you to turn right/left/climb/etc into Class A.
In Scotland the only Class A is between Aberdeen and the Scottish TMA (P600 and N864)....The route Jwoolard is taking is clear of overhead Class A....and the MEAs are all well within non-oxygen altitudes
I have tried to find out the conditions that cause UK ATC to discard validated IFR flight plans but nobody would reveal it.
Do you think it would help to put something in Section 18 along the lines of "full Instrument Rated" or similar....perhaps they actually read the full flight plan in the (relatively few?) cases that they review before throwing them out?
And presumably if/when the EIR is implemented an IMCR + EIR holder will have no restrictions on any IFR in the UK?? Other than perhaps doing approaches in Class A...which is only Heathrow? And I don't think that would be an issue for any PPL!
PS sorry about the multiple edits.....
I think that's quite a big question
And presumably if/when the EIR is implemented an IMCR + EIR holder will have no restrictions on any IFR in the UK??
is correct, IMHO.
I am in favour of the EIR for example but the case of the EIR-only pilot is likely to cause havoc with IFR ATC because where they are expecting a normal arriving pilot to seamlessly transition from enroute to arrival to approach procedures, they will now be faced with a pilot who is not allowed to fly a STAR (not even in VMC, and not even a Visual Approach because that's IFR as well) and who will be cancelling IFR before exhausting his enroute section.
His flight plan will have to be a Y or Z (probably Z if both ends are VFR, with IFR in between) with "VFR" filed after the last waypoint of the enroute section, and then perhaps a DCT to the airport, so while ATC will know of his intentions in advance, he may not be able to maintain VMC all the way down from enroute level.
But from a flight plan, how would they know if you have an IR, IMC or PPL?
They won't.
It would be the responsibility of the pilot to file, and fly, within his license privileges - as at present.
Sorry if I missed something in your meaning, but obviously ATC do not need to know if you are licensed to fly the route.
Re what Anthony was saying, this is my take on it:
An IMCR+EIR holder, flying say Bournemouth to Split, will have to file Y and with something like
EGHH (FL160)
SAM Y8 GWC DCT SFD Y803 LYD M189 DVR L9 KONAN L607 RUDUS L984 ASKIK Z74 KOSEK L603 OLETU Z106 BAVAX M867 ABISO L608 DOL L868 GISER L862 OKLAX VFR DCT
LDSP
There is an OKLAX STAR but he can't fly that, so will be cancelling IFR about 25nm before LDSP.
If he doesn't have an IMCR he will file Z and with something like
EGHH (FL160)
DCT SAM IFR Y8 GWC DCT SFD Y803 LYD M189 DVR L9 KONAN L607 RUDUS L984 ASKIK Z74 KOSEK L603 OLETU Z106 BAVAX M867 ABISO L608 DOL L868 GISER L862 OKLAX VFR DCT
LDSP
Bournemouth has a SID to SAM (implicit, rather than named SAMxx etc) which he can't fly under IFR.
Quite how well that would work, I don't know.
One funny bit might be where an airport of departure has several potentially applicable SIDs, and your enroute section could start on any of them, and then there is a question as to where you have to be VFR until 
And this is not purely academic, because if you do the VFR to IFR transition during a STAR, you will be illegal.
I am in favour of the EIR for example but the case of the EIR-only pilot is likely to cause havoc with IFR ATC because where they are expecting a normal arriving pilot to seamlessly transition from enroute to arrival to approach procedures, they will now be faced with a pilot who is not allowed to fly a STAR (not even in VMC, and not even a Visual Approach because that's IFR as well) and who will be cancelling IFR before exhausting his enroute section.
I would feel quite uncomfortable flying enroute in IMC, knowing that I can't fly an IAP at my destination airport...
When looking for the different options for instrument flying last year, I decided not to wait for the EIR, but just go for full rating. I'm glad I did it. One of the most useful privileges of the IR is the ability to fly an approach. No need to search for Visual Reporting Points or fly a circuit in dodgy weather. Just fly the approach and it will bring you down near the beginning of the runway...
where filing a low level route (say 5000ft) will result in the flight plan being chucked out at the UK ATC end and when you come to depart you get asked to contact London Info instead.
I have seen this done differently. Call up to activate your flight plan "I have you as VFR can change but there will be a delay"
