FWIW, I'm in the midst of a UK/France trip now, and here's what I've experienced...


UK
Getting a SIM in the UK was ridiculously easy. There are vending machines at Heathrow selling several carriers' SIMs. SIMs are also available at any Boot's pharmacy, Carphone Warehouse, and others. The best deal is Three, which will sell you a standard SIM or micro SIM for I believe 5 pounds (I happened to have bought one ahead of time on eBay). Charge it with 15 pounds of credit and you can get their 15-pound add on, which gives you 300 minutes of calls, 3000 texts , and unlimited 3G data for a month. The minutes and texts are only good for calling/texting UK numbers, not back home. The network in and around London was fine. Some dead spots, but nothing horrible or long-lasting.

I topped up my Three SIM at a Boots pharmacy in Heathrow with 20 pounds, got the 15 pound add on, and had another 5 pounds for credit to text back home (which I didn't use much, since I texted via Google Voice).


France
Getting a SIM in France was more of an ordeal. I first went to an SFR store, and they told me that their entire prepaid system had been down for a few days due to unknown technical problems, so they wouldn't/couldn't sell me a prepaid SIM.

Next, I went to Orange. The guy there flat-out refused to sell me a prepaid SIM for an iPhone, saying that with a smartphone I'd burn through all my data allowance in "five minutes". This despite the sign by the cash register showing prepaid SIM options, including an iPhone micro SIM with 200 MB of data. The flyer also said that it would take 48 hours for the data service to be activated, which is awful, so I didn't press the guy at Orange.

Finally, I went to Bouygues (pronounced "boog"), and they were happy to sell me a microSIM. He said it was 10 euro for the SIM and then I could buy more credit, but looking at my account online shows that that 10 euros is indeed reflected in my available credit. In any case, I got the "2 fois" plan, which allows free SMS in France and calls at EUR 0.45/min - a bit steep. There is a data add on of EUR 10 for 200 MB. Once you're out, you get an SMS from Bouygues and you can dial 630 to top up or buy another 200 MB with existing account credit.

Bouygues service is not the best. In the middle of Paris, I'd have inexplicable data outages despite showing full 3G signal. I frequently drop down to EDGE or get No Service intermittently, only to quickly bounce back to full 3G signal. They also block VPN connections. Still, it more or less works and is fine for short term.

Hope this helps someone.