I've just been looking at the international roaming set-up on 3, as I'm off to France in July. There is a choice of three different networks available for roaming over there, and it seems to imply that Bouyges Telecom is the preferred network as it is listed first, in its own section. However on further investigation it looks like it actually makes no difference which one you use, as all the services and charges are the same.

Anyone actually know if the only difference is the coverage or there's something more to it?

http://www.gsmworld.com/ is an excellent (and well maintained) site to give you the latest coverage limitations and maps of all the GSM roaming partners worldwide.