Ask HN: Which 3G dongle to use in UK (and London info)?

I'm traveling to UK (Rumford, near London) next week and need Internet access for my Macbook Pro. Which 3G dongle should I use, since I'm clueless about the situation outside US. I'll also buy a pay as you go SIM card, which one should I buy.

If you know the London area any other information (pubs, cool places to see, beer, how to greet royalty if I see them, etc.) would be much appreciated.

Edit: Thanks for all the responses. Yep, it's Romford, not Rumford :-) Also, is having free WiFi common in UK stores, like in the US, e.g. in Starbucks?