While I don't dispute that for you, and the places you visit, O2 have given you good coverage. They're overall 3G coverage in the Uk is the worst of all the networks. I would avoid them like the plague, unless you plan to only visit cities in the UK, and not use the iPad much when travelling between these cities.
This article showing off com (UK Regulator), while being slightly out of date, gives a good indication of how the networks fare for 3G coverage:
http://crave.cnet.co.uk/mobiles/ofcoms-3g-coverage-maps-o2-embarrassed-49302921/
As a past O2 employee, I still feel a little loyalty towards them, my personal mobile and all of the voice connections my family has are with O2. They are not subsidies connections either.
If you're going to chose a network in the UK for 3G coverage, stick with 3 or Orange. They have a far better footprint than the other networks.