the russians could quite legally have flown along side you taking pictures!!
They did quite frequently, we used to formate with the Bears.
What I was trying to say re the FIR is that normally the rules that apply within it are those of the controlling authority though they don't have any territorial claim to it. We regularly used to fly the FIR boundary and talk to nobody. I stand corrected on the Limit of Territorial Airspace which makes sense.
If you are over the high seas outside controlled airspace I don't think anyone is going to be concerned if you are using your UK IMC rating.