Tescos, asda and boots all do size 6s, and I think that the Easy Ups that Pampers do come in 6s too.
I found them useful with Earl as he got too big for size6 normal nappies but the pull ups seem to be a bit more stretchy lol. Also, for night-time when potty training they were useful - in the day he was in pants but wasn't dry at night so needed something. The pullup was handy as he was in something different to nappies (I got him Huggies pull ups with the Cars designs lol).
Eddy is currently in pullups in the daytime as an experiment. I did the same thing with Earl when he was about 12-15 months. He simply will not lie down for more than 10 seconds at a time so being able to do nappy changes in a more upright position is handy lol. At night they're not as absorbant, so have normal nappies then, but in the day nappy changes are no longer a 2 man job lol