Could I get a three SIM, top up with £10 and then get 1gb of data, since their 321 plan charges 1p/mb (1,000 x 1p = £10, 1,000 x 1mb = 1gb)? Or somewhat less than 1gb plus a few phone calls?
http://www.three.co.uk/Store/SIM/Pay_As_You_Go
This would be better for me than the All in One £10 add-on, since I likely won't text or make many phone calls, but may use more than 500mb of data.
Could I then use it in France with their free roaming?