Hitchhiking in the United Kingdom easily possible, although the British are a little surprised to see people still doing it these days. Hitchhiking is illegal on the motorways, but is allowed at service stations and slip roads. When travelling a long distance on the motorways it is best to stick to the service stations as getting a lift on a slip road will take an exceptionally long time.
Useful tip: In city libraries you can use Internet PC's for free after a short free registration (some want to see a passport, some aren't that strict).
If you're hitchhiking long distances and considering making a sign, in general people use the motorway names rather than the city names. For example, from London to Cornwall have a sign saying 'M4'.
Hitchhiking from England to France is not too hard. Just hitchhike to Dover.