The UK has many advantageous prepaid sim offers...may I refer you to
www.yourcallworld.com which explains how you can make calls in the UK to other country's landlines for as little as 3p/minute (although calls to mobiles outside places such as the USA and Canada are somewhat more expensive) using a T Mobile UK sim card (it also explains how to obtain a T Mobile UK sim card for next to nothing)...along with it comes free reception of calls while in the UK. That is probably as of today the best deal in the UK.
The big 3 in France are pretty expensive but at least calling the UK isn't terribly bad as they do subscribe (of course they had little choice) to the new eurotariffs...whichever you choose you get the free reception of calls while in France (what is it, 0,32€/minute or somthing like that to make calls from France within the eu or you can go the direction suggested by Mr. Ed who I believe works for one of the international sim companies; the only downer in this is that you would get a +44 (although not UK, considered by most telcoms to be UK) number which might not be reachable via yourcall world. Also as noted, the French sim cards come with very limited validity and they steal your credit after very limited periods of time (this is something the eu should be on) so if you go to France annually, you have to get a new sim card every year.
I don't know if it is true that vodafone coverage in Scotland is so much dramatically better than T Mobile UK that its much higher prices to call internationally are worth it; only a British "expert" can answer that for you......so as of today, and understand it can change tomorrow, probably the best solution is a T Mobile UK pay as you go sim card in the UK using yourcall world for out of country calls, and a French payg in France (knowing full well the limitations of the French carriers).....
What a ramble.