I use blueyonder which is the telewest cablemodem service equivalent to NTLs cablemodem service.
It depends on your needs, but if you are currently using 128K ISDN and your requirement is internet access , rather than hosting servers, then I would recomend cable. you get a dynamic ip , but this isnt an issue as you can use dyndns.org and the esmith to keep your dns entry uptodate.
I believe services are currently about £33 per month for 512K and there are faster packeges to come.
Alternatively you can try
www.easynet.co.uk theyre cheaper than BT, give you a pure TCPIP connection with upto 13 static IPs for 79 per month for 512K or 99 pm for 1Mb
good luck