A friend in South Norfolk used them but had a lot of customer service issues and poor response to enquiries. As you are in East Anglia I would recommend local company Comms Consulting / Comms Supply (based in Norwich) who can usually not just obtain a substantial saving on your current costs but preserve or exceed customer service levels.
I have no vested interest in recommending them, but I use them for both work and home (as I often work remotely) and they have been very good indeed, including wrestling witth problems caused by our existing BT circuits (we still have to use BT for a few critical circuits at present) and an office move, and also the fact that some of our lines are critical for the safety of our service users/customers.
My personal experience in more rural areas is that many companies simply do not want the more "complicated/demanding" business customers who make extensive use of internet/broadband and though they might intially take the business (sometimes because they don't even know where East Anglia actually is or the challenges facing telecoms providers with regard to extra maintenance costs of rural area circuits) struggle to provide an acceptable level of customer service.
We've had other companies openly turn down business from my employers (a medium sized business mostly based in rural Suffolk) simply because we've got slightly different voice circuits to a bog standard BT line and they didn't want to deal with a bit of extra admin work that BT demanded (it wasn't even BT not offering the service they would have needed!)