I have two brand-shiny-new SIM cards for UK network "giffgaff" which is the prepaid arm of O2. Free except for the postage. First some first served, PM me.

These are ordinary multi-size SIMs, just punch out the size you need - and then you go online and activate them using the code printed on the SIM, then it tells you the phone number and you can load credit etc. The credit doesn't expire, and the phone only needs to be used once every 6 months to keep the number alive. Perfect if you live in NZ and visit the UK once in a while and want to keep the same local number each time - drop the SIM into a phone here and let it roam (works - I've tried it). Just send a text once every 6 months from here to keep it alive.

Yeah you can get these on arrival at the airport in the UK but I arranged to get some sent to me here as I was arranging a big UK trip which needed a local phone number for some of the events I was planning. So I got them sent to a relative there, who then forwarded them to me. They don't normally send them to non-UK addresses

I used mine extensively in the UK on my vacation up there this year and they worked fine - 3G speeds are nothing to rave about due to population density, we're spoiled here - but at £15.00 for a "goodybag" which lasts a month and gives you 5G of 3G data, and unlimited texts and 500 mins of calling, the're fairly inexpensive.