The move went without any hitch or hassles at all. I was off-line for about 10 minutes, and the connection has been rock solid. I am very pleased so far that I made the move.
BT refunded me 99 pence. I wasn't sure if changing to a LLU provider constituted changing provider "by the normal process" which means that the 30 days notice period is reduced to 14 days. Anyway, clearly it is as I was only charged by BT up to the migration date, and their final bill was actually correct.
It was a remarkably simple and trouble free change; just as should be.
I had better go and change my signature though
BT HomeHub 3
Core 2 Duo E7500, Asus P5Q Pro turbo, 4 GB Ballistix 6400, MSI GeForce 8600GT, Corsair TX650W PSU, APC 650VA UPS