Sorry for the lateness in replyin.
I have all the 4 main network sim cards (Sad i know)
However results from what I have found when traveling about the uk.
o2 came out tops for 2g coverage. Hardly lost the signal 3g on o2 is sort of none exstance, even now. but i have noticed it becoming wider and better.
Voda and orange had mixed results. Orange was good in towns but patchy in the country sides. Voda was ok to good in towns, but better in the country side.
3g on orange is very good, with the signal in more places than ever.
3g on voda is ok, but no where as good as orange
T-mobile seem to have improved alot. boh the 2g and 3g coverage were good, but still not as good as the above other networks.
However agian the networks vary from one town to a other. What may be good in one house the other network may not be good.
so its always better to find out first