Most UK companies supply a VOIP service. If you want all the PBX features from the company you are looking at a centrex service. BT and most of their competitors will be able to offer this, but the chances are the cost will be very high.
If your requirement is for more than 8 lines then look at having your own IP PBX. There is an open source IP PBX called asterix. It will run on fairly modest hardware.
If the requirement is for 8 lines or less that checkout BT's broadband voice offering for business. If taking this out and the supplied unit is a 2wire 2700 make sure the firmware is 6.1.1.48 or above.