I believe that I can answer my own questions now that I have finally received my iphone4.
I called up 2 degrees and they sent me a microsim a few weeks ago with no hassles at all.
Once my phone arrived yesterday, I gave them a call and they transfered my account over to the new sim then and there. It was up and running in less that 5 minutes.
Had a fiddle about with the settings but finally got it to access the internet over the cell network.
One thing though, it shows up as vodafone on the carrier display??
Anyway I activated 3g and low and behold it seems to be using the extended 3g network 900mhz. (I am in Cambridge, so not 2100 3g).
I did a speed test and got 960kb/s download speed which I am happy with. As I thought it may drop down to GPRS speed.
So at the moment, very happy.
Coverage is marginal so the old grip of death would make it drop out very easily. I have fitted a "best skins ever" full protector kit (front, back and sides overlays) and a Belkin shell which has stopped this happening.
So to wrap up, I can say that it does seem that 2 degrees do use the extended 3g setup that vodafone has on their network.
Cheers