Buy an ordinary DECT cordless phone which has a standard telephone jack to connect into a POTS line (like any standard telephone). USB Skype phones are probably not compatiable with software which supports SIP.

So once you have your telephone, you need to plug that into an ATA (Analogue Telephone Adapter) which basically converts a digital voice over IP connection into an analogue signal which the telephone can recognise and operates as if it was plugged into a standard telephone socket.

As far as ATAs go, I would recommend the Linksys PAP2 (available from http://www.broadbandbuyer.co.uk at a respectable price). Make sure you don't get the one locked to Vonage as it cannot be used with other providers - Unless you want to use Vonage of course!

All that is left to do is to plug your PAP2 into your network switch or router using a UTP patch cable (network cable) and plug your telephone into the PAP2. I believe you can also plug in your standard telephone line as a backup.

So what you should end up with is:
Router -> Linksys PAP2 -> Standard Cordless Telephone

Remember to purchase any UK adapters for the Linksys PAP2 if that is what you choose to buy! These should be listed on broadbandbuyer.co.uk.