I'm no 'three' shill, but the three PAYG prices are so much lower than the others, that I'm surprised anyone bothers with anyone else.
Don't forget you can add montlh;y bundles to that, with no contract or requirement to renew.
When I last moved, I bought a £15 "unlimited data for a month" bundle, as I was without a landline for a couple of weeks.
What's more, I phoned them once regarding my address change, and 'sales' tried to get me on contract. I worked out that even their best 'specials' were worse than the PAYG option, and I told her that.
Of course, she was surprised(!) at their PAYG rates, and I ended the call suggesting that if they want more sales, they should ask their PAYG department to stop undercutting them!