Well, you can buy a VoIP phone, which is a device with an RJ45 network socket. I've never tried one of these myself, but hear they are better than using an ATA with a 'normal' phone.

Myself, I have a Linksys PAP2 used with mainly a Panasonic DECT, the Panasonic works very well because it has name and number CLI, so gets name field from VoIP calls and shows it.

Around the rest of the house i have mostly BT Diverse 3000's, no text CLI, but number CLI works fine.

Any phone that works well on PSTN should work just as well on VoIP. Only problems with VoIP are reliability issues, you need a good stable internet connection, and a stable provider.

It is not possible to stop paying BT line rental, unless you can get broadband from a cable provider.