Last weekend I was in PC World where I came upon a Philips cordless VOIP phone, but it had a major fatal flaw - it was a USB device. What I want is cordless VOIP phone that has an ethernet port and uses a protocol supported by Asterisk. Today, I discovered such a device exists - The Siemens Gigaset C450 IP, which will be the C460IP in the UK.
I'm not sure what the differences are between the 450 and 460 that merit what appears to be a ~£30 price difference (you can probably guess which is the more expensive model). I'm betting the only differences are UK Caller ID compatibility (it's a "dual mode" phone that can connect to the landline) and a power adaptor and phone line with UK plugs. If those are the only differences, I'll have to see if I can source the C450.