The UK has probably the most deregulated telecoms market in the world. You can simply walk into a '3' shop and purchase a pre-pay SIM and you're good to go. They don't want to know any personal details, addresses, telephone numbers or what you had for breakfast - they just want your money - cash or credit cards are fine. On a '3' SIM roaming is disabled by default, but you can request it be turned on and then it will work fine in France - however, you'll be charged at roaming rates.
A '3' card can be topped up on-line even if you've used your entire data allowance because '3' always allow a '3' SIM to surf to the '3' website even if the allowance has been used (and surfing the '3' site is not charged, so you can check how much data you've used at any time without incurring a charge for that checking process).
Another advantage to '3' (no, I don't work for them!) is that their network is entirely 3G - they have no 2G basestations anywhere, so you don't find yourself on EDGE or GPRS - it's 3G and HSDPA all the way.
Tim