IFR clearances at “wrong” levels, i.e. say FL70 instead of FL80 when going westbound, as you will know, are very very common (every day). Most ATC units don’t care and that may make sense in class D or higher. All aircraft are controlled anyway, so there is no collision risk.
IFR clearances at +5 levels? Never had that either. But I can imagine it happens, when
a) a pilot presents himself on freq at say FL65, saying he IS IFR (which, in this case, he can’t be, but anyway).
b) the pilot requests the transit at FL65, IFR
c) the airspace is pretty empty at that moment
Then, the controller might not care and might clear the aircraft without having him change level. A “fault” by the pilot, which ATC did not bother to correct.
I requested that transit at FL065, citing icing conditions below (IIRC I said that, but it was actually a fact). They could have told me to climb FL070 (080 actually for that direction) but didn’t.
I have also done IFR flights at funny levels when advising ATC that I am at the aircraft operating ceiling, and there is hazardous wx below. Whether they get difficult about it depends on where you do it. With a particular well known controller in the Munich area, you will have a bit of a struggle 
Elsewhere not a problem, if you explain.
Usually, at typical light GA levels, there is nobody else around.
Surprising that we don’t appear to have much illumination of the original Q.
At FL65, you were not in Solent CTA at all, but in Portsmouth CTA. So you weren’t in any airspace controlled by Solent Radar, but in an airspace controlled by London Control. So, the clearance you received from Solent was not issued by them, but actually relayed, and issued by London Control.
How would a normal pilot know that? It would be doubly curious that London Control would authorise IFR at FL065.
Qalupalik wrote:
This is possible in UK under the conditions of a general permission. See ORS4 No. 1126 of 13 Aug 2015 (link).
I don’t think that that is applicable, as it applies to
Peter is clear that he was in CAS.
In the US they call it “VFR on top” (they mean IFR on VFR level)
Timothy wrote:
Is it a common thing?
I’m pretty sure I got an E→W transit of the N-S class A airway (on a filed VFR flight) that passes through Exeter a couple years ago. I asked quite a long time in advance.
Yes you can, but you need to have an IR to be legal. I’ve done it a number of times. There is rarely any traffic in it, at “our” levels.
But sometimes they cannot coordinate with whoever owns it for ages, and one has to dive down below it at the last moment. It’s unpleasant if you have to dive down into bad wx and turbulence, with a passenger who gets scared.
boscomantico wrote:
IFR clearances at +5 levels? Never had that either. But I can imagine it happens, whenOn one occasion I had asked for “lower due to icing” and the controller immediately offered a descend to the MVA, which was something like 4300 ft. I continued the flight at that altitude to my destination. So if there is a good reason, they will accomodate you.
Timothy wrote:
I don’t think that that is applicable… Peter is clear that he was in CAS.
To the extent that the scenario in post 41 involves cruising flight under the IFR outside CAS, precisely what the first post depicts, the general permission may apply subject to a condition in subparagraph (b). There is insufficient information to support the statement I quoted.
Peter wrote:
Surprising that we don’t appear to have much illumination of the original Q.
I’m not sure we will know unless someone call Southampton ATC for a chat.
Or someone volunteer to reproduce the flight but doesn’t descend, then potentially use a lawyer, …
Maybe, with hindsight, you could have ask the controller if he is able to clear you or able to coordinate before stating that you’ll be descending. It would have been interesting to know the reaction of ATC.
There are loads of Letters of Agreement between adjacent ANSPs. Like MATS Part 2, they are not public. LoA are used to lend and borrow airspace between ANSP sometimes long term, sometimes very depedent on timing or runways in use, ….
So we can only ask. I don’t think any pre-planning would give us the good frequency.
