As mentioned above, the update expected today or tomorrow is supposed to offer a way to turn off EDGE.
Otherwise, if you set your email to check manually and don't open your mail app or any other internet apps, you won't be hit badly. I was just in England, and I bought the AT&T Global Data plan ($25/month for 20MB of international data) before leaving, but I was still very wary of running up usage. I tried to use the internet apps in WiFi zones only, and after a couple of days I'd only had about 200k of EDGE usage (track this by resetting your usage counters when you arrive). After that, I was more free with my use of the phone, and I ended up at about 10MB after 5 days.
As an aside, London is rife with WiFi zones, but most of them you have to pay for. The City of London has "The Cloud", which is supposed to blanket the entire "square mile", but I found it patchy. The bottom line is that you'll probably end up spending more money on WiFi charges than on the AT&T roaming plan.
