Everyone will be thinking this, but someone may as well say it.
FPV flyers are not really much of a priority for ofcom it seems. If you were transmitting at 500mw for days on end, or permanently, and someone complained, you might get in trouble. For ten, or twenty mins of use a week, and in different areas all the time, it is very unlikely you would get 'caught' or if you did, weather they would care or do anything about it.
I deal with radio equipment in day to day work, I design and build radio devices that have to go through CE and FCC testing, and I have some competitor products that are illegal to use over here, there are thousands in the country and they have been around for about 10 years. No one has ever been caught, and they are sitting on the 900mhz band, which is what mobile phones use - far more likely to get caught abusing that band than you would on a 5.8 frequency.
People can make their own mind up, and just to be careful, I wont say what system i'm using, but all I will say is, I wouldn't take the laws too seriously or be scared of arrest because of an overpowered FPV transmitter. There are far worse things for Ofcom to be dealing with (I also believe that the police cannot do anything about radio misuse, only Ofcom has the power to deal with this.)
I know a guy who has a 300w (not milliwats, 300 full watts) of power on a CB antenna on his car. Loads of people do it, and have been forever. Thats dangerous power that can (and does) interfere with alarm systems, TVs etc. Ofcom dont seem to care about that either....