Entwood - when I go away, be it tents, mobile homes, cottages etc, we tend to stay in th countryside. I have no desire to stay in large towns/cities.
I tend to take a GG sim, and my works Voda sim, for use in the dongle/mifi. Invariably, it seems we end up using the GG sim (bit of a bugger - I have to pay for that

) due to coverage. O2 doesn't have great 3g coverage, but then nowhere does in the sorts of places I stay in. O2 does, invariably, have 2G coverage, which is usually usable for browsing and email, but hopeless for anything else, such as streaming.
Also with O2 for data, it helps if you have a 900Mhz 3G device, as O2 have a god amount of 900Mhz 3G. Earlier in the year, when at Haven with the foogies, I nipped down Tesco and bought a newer O2 dongle (E173 - my old one was E170) to handle 900Mhz 3G, which was available in the area. My home town, Brackley, is 900Mhz 3G as well.
I tend not to use the dongles any more, I tend to use the MiFi unit (MiFi is a phone sized device that acts as a wireless access point, but using cellular to provide the internet).