Answer:
It is the sim card that relates to weather your phone is 3G or not, BUT- - - you must be sure that the handset that you are using is indeed 3G compatible! As a rule, virtually all new handsets are all 3G compatible, it would only be pretty old handsets that wouldn't be.
The uk really should have 4G up and running by now, but sadly, we are lagging behind other countries in this area. Some parts of the uk use H+, which in basic terms is kind of like 3G.5.
I hope this helps.