Legally speaking, even if (as opposed to ICAO standards) you need permission to cross a D-area in the UK, can the UK prosecute someone who flies through an active D-area in a UK FIR, but over international waters?
I expect not. In Swedish FIRs (Sweden does follow ICAO standards in that it is always legal to fly through an active D-area) there are no R-areas over international waters, only D-areas. In some cases you have an R-area inside the border and a D-area outside it. The only reason for that arrangement I can see is that R-areas over international waters are unenforceable.
That does not mean that flying through an active D-area is always a smart thing to do, of course. But it could be if you know there is no activity in the area.
I think you’re right. UK AIP ENR 5.1 helpfully lists the Statutory Instrument you might be breaking for each D area. There are none I can see listed for the areas in international waters.
Hmmm…… not many people knew that! 
These areas are carefully monitored both by the UK and France – that they go to 55,000 feet gives some hint at what might go on inside them. They are inactive at times, you just need to speak to Solent or Portsmouth and get a crossing service.
In fact, the majority of UK Danger Areas is not subject to any byelaws. Only about 10-15% of them.
I usually speak to Plymouth Mil or sometimes London Info will co-ordinate a crossing service.
It’s even easier than looking in the AIP – the UK ICAO charts have the ones with byelaws prohibiting entry marked with an asterisk.
If there is a crossing service available, there is a – wait for it – cross next to the designation.
I agree they should be restricted airspace it is rather silly this way.
