mtbmatt – sounds like you’re doing much the same as me.
Family Stoner off to France for a month – we do so every year.
I take laptop and continue to work from the back of the van as and when I need to.
We used to be with O2 from whom you could buy blocks of roaming data in advance (200Mb for £50) which was good VFM.
Then we moved to Voda and it’s daily rate was a bit pricey, so I picked up an SFR card pre loaded with €50 of data that I put in an unlocked MiFi Wireless dongle. I use the wireless dongle with my netbook, but still let myself use voda roam on the phone since fat downloads Id do over a local SIM.
This year will probably do much the same even though I can tether to my HTC now. I have kept the SFR SIM, if its still valid I can charge it at a shop. You can get even better data SIM deals from hypermarches.
EDIT if you get a dongle, you may need to make sure its unlocked so you can use whatever SIMs you want. A voda dongle might not let you use an SFR card.
PS I think Auchan supermarket used to do v cheap SIM deals if you can find one.