@h15t0r1an, am I interpreting Three's website correctly that I could do this:
Buy a SIM and a £10 credit and use it in the UK at 3-2-1 rates. Just before going to France, I could buy a 500mb add-on for £5 and use it for data in France, plus calls and texts at roaming rates (obviously provided I don't use more than a total of £10). If I just use data, I could use a total of 1,000mb (500mb in the UK at 1p/mb plus the 500mb add-on in the UK and France).