An update to the prices I previously posted (added links & names of the plans;Orange UK plan changed from unlimited text to 600;Orange EU plan extended offer to Sept 30;removed tmobile UK because plans are unclear):
In UK:
3 All-In-One 15 Add-on - £15 = 300UK min, 3000 text, unlimited internet
giffgaff - £10 = 250UK min, unlimited text, unlimited internet
Orange Dolphin - £15 = £15 credit, 0UK min, 600 text, 500MB internet
T-mobile - <not clear>
Virgin Mobile Addict - £15 = £15 credit, 0UK min, 3000 text, 1GB internet
Vodafone text & web Freebee - £10 = £10 credit, 0 min, 500 text, 500MB internet (call prices the same as Vodafone Simply plan)
Vodafone Simply - call-25p/min, 12p/text, £1 per 25MB/day (up to 125MB, then 4p per MB after that)
Roaming in France:
3 - make = 28.8p/min* (higher to some regions) receive = 7.9p/min Data = 69.6p/MB
giffgaff - make = 28p/min receive = 7p/min Data = 69p/MB
Orange - make = 28p/min receive = 7p/min Data = 59p/MB or 4MB bundle for £5 (which is about 2x the per MB price!)
Orange EU Combined Bundle (through Sept 30; plan good for 7 days) - £15 = 50 min, 100 text, 30MB internet
T-Mobile standard roaming & Travel Booster (data) - make = 28.8p/min receive = 7.9p/min
Data = £10 for 50MB
Virgin Mobile + EU Data Travel Pass - make = 28p/min receive = 7p/min Data = £10 for 25MB
Vodafone Data Traveller &
Vodafone World - make = 28p/min receive = 7p/min Data = 69p/MB for first 2.9MB, then no charge until 25MB, then 69p/MB after
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The Vodafone Simply option without freebees seems like it might be a good one unless you are a heavy internet user (in which case you would definitely need to look into the "fair use" policies of the carrier to see if they'll cut you off or throttle your data service after some point). Also great for just a short trip.
Another thing I noticed is that you might have to do things in a specific order for some of these PAYG top-up offers. For example, read the fine print on the Vodafone text & web offer. Unless you opt-in for that freebee first, you won't get it when you top-up with £10. For the T-Mobile plan, they don't have clear terms of how much data you get for your Internet booster. Plus the bonuses they offer on top-up don't appear to apply to the current month but the next month, making them not valuable to most travellers.