Bookworm,
Thank you, I sit corrected. It is indeed a maximum of 12nm these days.
Whopity,
You may have been operating over the high seas. In fact you would have been doing so if you were more than 12nm from the coast. Therefore, you could do as you please - and so could every other military operator i.e. the russians could quite legally have flown along side you taking pictures!!
Fuji Abound,
If you look at the ICAO Convention Article 1 and 2;
http://www.icao.int/icaonet/dcs/7300_cons.pdf
and then look at the UN Convention on the Law of the Sea
you will see exactly what Territorial airspace means.
FIR's are simply for the coordination of ATS and are formed by agreement. There is absolutely no way that the UK can claim that the airspace at 54N and 29W is UK Territorial Airspace. They do however, provide an ATS service there under an international agreement. (Shanwick FIR).
So the problem is that the limititation on the IMC rating to only flying in IMC within the UK prevents anyone from flying in IMC with the rating over the high seas!!