There are quite a number of ISPs in the UK: BT, Virgin Media, TalkTalk and Sky, Zen are some of the biggest.
The one I like the most (and I am quite biased on this choice) is Claranet. Claranet is actually an MSP (Managed Solution Provider) and not an ISP (Internet Service Provider), although they did start as an ISP.
Claranet is very reliable, especially the SOHO (Small Office Home Office) Team based out of the Warrington office near Manchester.
Claranet provide ADSL, FTTC, FTTH, FTTO and Leased Lines as well. And that is just their connectivity products.
A dislike would be cost - most of their connections cost a bit more than the other ISP; Claranet is very much aware of it and they freely admit that they are not out to compete on price, but prefer to be slightly more expensive and focus more on customer experience and satisfaction.
A limitation is that Claranet is somewhat dependent on OpenReach (as do all the other ISPs - kind of an open secret) and on the services OpenReach supply in that area. OpenReach pretty much have a complete monopoly on the telephone infrastructure. The only way to bypass using the OpenReach monopoly is to use Virgin Media's infrastructure which is roughly 99% their own which supplies FTTC and coaxial from the Cabinet to the premises and they have been deploying 200Mb/20Mb connections (which actually deliver the speeds advertised). On occasion Virgin Media will use the OpenReach infrastructure for areas where they have not rolled out their own infrastructure.
For the CPE, you can purchase it directly from Claranet or supply your own. The equipment purchased from Claranet comes with standard preconfigured settings so its pretty much plug it in and it connects and works, straight out the box. If you wish to configure your own equipment and are struggling with the basic settings, the SOHO Technical Support team are able to help you configure it over the phone.
Whenever I have a technical issue, the waiting on the phone is short (anything from 15 seconds to 3-4 minutes) unless there is a major outage, then the wait time is anything from 4 minutes to 25 minutes although it would be exceptionally rare for it to be longer than 10 minutes and the outage is usually affecting services other than the broadband.
If anyone is looking for a really good ISP/MSP for a home or office internet connection, I would not hesitate to recommend Claranet.