In the UK and France (and most other countries) most hotels, malls, coffee shops, restaurants, trains, buses etc have free WiFi available. For example, in the UK, Starbucks, McDonalds offer free WiFi. Most hotels it's either free (get a code from reception) or you can pay a nominal fee. This obviates the need for 3G entirely. Of course, if you need connectivity when you're 'out and about' this doesn't help.
If you're going to sign up for PAYG in the UK, I'd recommend the provider '3'. They have an all 3G network - so it's high speed everywhere. Most of the other providers have hybrid 2G/3G systems - with 3G only in urban areas. This discrepancy is due to a quirk of the licensing conditions - '3' were a late entrant and missed the original 2G round of licence approvals. You'll find cellular coverage is excellent in the UK, with very few blackspots. This, again, is a quirk of the original licensing provision. In 'the olden days' the UK had a nationalised phone system with a 'public service' provision requirement. So, when the original cellular licences were awarded the bidders were mandated to provide a specified coverage - including remote rural areas.
Although the UK system is now fully private and entirely deregulated, that 'public service provision' mindset is still around.
I'm not sure why it's so difficult to casually get a PAYG SIM card in France. Perhaps some French members can enlighten us but you'll find posts from several Members who, when touring in France with their iPads, gave up trying to get one there.
Tim