Just go to a 3 store and buy an ordinary SIM. It'll cost 10 pounds, but that includes a topup voucher for 10 pounds (actually about 10.62 pounds--I think that's due to the reduction in VAT from 18 to 15 percent).

You can either apply the topup to the laptop data deal (you can only do this for the whole amount of any topup), or you can just apply it to regular phone use. However, with the regular phone deal, you can also buy 1 week of data for 2.50 pounds, and reduced rates for longer periods. Unlike the the laptop deal (10 pounds for 1 gb over a month), this data is intended for smart phones and is limited to about 50 mb a day (1.5 gb a month--go figure). You have to register at my3.three.co.uk and tinker around there to buy the data addons.

This works fine with a regular 3G phone, which I connect to my laptop via Bluetooth. Note that it has to be 3G: 3 does not have a GSM network. The APN is three.co.uk, so typically set the laptop to send AT+CGDCONT=1,"IP","three.co.uk" and to dial *99# (or *99***1#).

The 3-like-home thing works fine: I bought a week of data in the UK and used it in Italy. Note that Australia is not included in 3-like-home for prepaid (only for postpaid).

Don't try discussing any of this with the kids in the store: they won't have a clue what you are talking about. You need to know enough about phones, SIMs, and Bluetooth DUN, to set this up and get it working all on your own.

T-Mobile UK has prepaid data on GSM: just text WEB to 441 and you're good for 5 days (at about 40mb a day) for 2.50 pounds. If you don't send the text, it's 1 pound a day. The APN is general.t-mobile.uk (notice it's .uk, not .co.uk). A UK T-Mobile SIM will not work in a US T-Mobile phone: you need to unlock it.