A bit of transparency would be good...
... surely the carriers / MVNOs could be forced to be a bit more open about what the agreements are - the consumers should know what they are getting - do they sit in a 'tier' on the network and what tier it is. Then the consumer can make the decision as to whether we want to pay to be in 'higher' tiers - be it PAYG or contract, home carrier or MVNO.
There are clearly people stating tiers don't exist, but all I can say from my own personal experience with O2 PAYG (my usage being light data - browsing, etc.) - in the same locations: O2 was OK, giffgaff was consistently poor (almost unusable) and Tesco has been OK as well. Others will no doubt say the opposite, but the networks should tell us what is going on.