Ask HN: Data-friendly, low-cost mobile networks in the UK?

Hello, I'm considering moving to the UK and would like some words of advice on thrifty mobile networks. For example, here in Spain I pay Simyo just 7€ + VAT for 555MB of data and pretty cheap call rates: 0.15€ connection fee, and 0.05€ per minute. (The minimum payment is 7€, so you have to talk/browse quite a bit before you exceed that.) Moreover, since my phone is unlocked I don't have any binding contracts.

I was hoping the market would be at least this competitive in the UK, but I'm having a hard time finding anything this thrifty.

Any feedback will be greatly appreciated.

Thanks!

Edit: I'm now looking at Lebara's offerings, which are OK... but I'm disappointed that tethering is prohibited. Simyo encourages tethering and VoIP.