Alternately, on the iPad you could choose Boingo Credits...

http://www.boingo.com/wifi-plans/mob...-wifi-credits/

...this is a prepaid plan where you pay $1.99 per hour or $19.99 for eleven hours. The hours do not expire which means you can carry them forward to use on other trips, and there is no automatic renewal or cancellation issue. In my experience Boingo Credits work well if there is a Boingo SSID, but not as reliably when using partner sites. However, if the app does not recognize the partner SSID a call to Boingo Customer service will typically fix it (eg they will authorize you to purchase a pass with a credit card and issue a credit to offset the expense.)

This said, free wifi is very easy to find in London, in you drink coffee

See here, for example:

http://www.thecloud.net/free-wifi/