Bear in mind 3 will not be an option as it mainly 3G tech based, which the quad band phone will not be able to address.

That said you've got the majors (Vodafone, O2-UK Orange, T-Mobile UK), and the Virtuals (Virgin, Fresh, Tesco, Lebra, etc).

Again, as graraps said, you can pick them up cheaply at at newsagent (I'm paying 99p/card at the moment - I'm testing a unlocked quad band phone here).

Coverage in London and stratford is pretty much blanket 2G coverage (with minor local variances). Again, check the network maps

If you're buying in the UK and want the reassurance of the bigger shops, head to the network shops (all the networks have shops to buy SIMs from) or a Carphone warehouse.

As there is a group of you, you might want to all stay on the same network and set the tariff up so you can take advantage of network-to-network calls (eg T-mobile network to network calls are 15p/m on one plan, yet on an everyone plan, that same call is 25p/m).

In reality, you can't go too far wrong. Alternatively ask a UK flyertalker... some of us get 4 SIM cards for filling out a form via QuidCo which can be sent onwards....