
It can depend on several things. Firstly, you have to consider where you are in relation to the transmitter. This doesn't just mean distance but also are there any steep hills as in Lake District/Peak District proportions? If you have anything like those between you and the transmitter that would make a difference. Secondly, do you have Pylons very close to your house? They can interfere with signals. Thirdly and most likely, it could be the base itself that is interfering with the signal. Ask O2 if this could be the case, they should know, but the assistants inthe shops are a bit hit and miss. If someone you know who is on Vodafone is only getting one bar I would say it is probably the base you are on. You could always ask Vodafone whether the base would make a difference to signals if you are not happy with O2's answer. Also, being that the base is forces it might be the equipment that the base is using interfering with the signals.
Sorry rambling a bit but hope it helps.