Am I right in thinking there is a benefit to using 2 separate ISPs?
Most of the problems seen are ISP related and at the ISP end of the connection - not at the exchange.
BT Wholesale seems very reliable to me, but I would not use BT retail as a provider. (I have had no trouble with either Zen or Entanet)
If I had a reliable LLU provider at the local exchange then I would use them for the backup connection.
Alternatively, if you have a cable connection available, then I would use that for the backup.
Line One:- Zen - DrayTek Vigor 2600VG
Line Two:- EntaNet (Aquiss) - DrayTek Vigor 2600