I agree with getting PAYG SIMs, definitely do that. I got PAYG SIMs with all networks and tested it, found Orange was the best and as I was already on Orange I just upgraded.
I did want to use Three but their indoor coverage isn't great in a lot of places and it was just not even an option where I live as there is just no signal at all, no 2G backup and like 1 bar of 3G when i'm upstairs if i'm lucky.
Again, that's just me, I like the idea of Three (Fast speeds, AYCE data, huge 3G network) but for me the signal is not very good in most places because it's 3G only, especially at home. Again don't think i'm bashing Three because I am saying they are very good in a lot of ways, Just not for me.
But definitely get loads of PAYG SIMs from all networks and test them out - you don't need to topup or anything for them to receive a signal.