British Isles is a bit vague... for example Dublin is in the Republic of Ireland (which is not "british" at all, don't ever tell them!
), while London or Belfast are in the United Kingdom, two different countries with different operators. There are also other "almost-countries" like the Channel Islands or the Isle of Man which have different operators as well, but probably you are not going there.
I think that probably 3 could be a good solution with the "3 like home" option, since it has coverage both in EIRE and UK (although not so good...). Isn't O2 doing something similar as well? I ask to our British and Irish friends for more info
