It would be prudent to extract your MAC code from BT early. They can take longer than the legal maximum of 5 days.
I recently left BT for PlusNet (totally racked off with BT Business Local and wanted Annex M too). I signed up with PlusNet and then asked BT for a MAC code.
BT failed to supply my MAC code within 5 days, on day 6 I went away for a week and on my return extracted the code from BT by telephone and supplied it to PlusNet.
PlusNet set an activation date of by midnight 7 days on from my supplying the MAC code and soon shipped my router. BT ADSL had stopped working by that day (I had been away again), but PlusNet ADSL was only enabled during the telephone call I made to them around 18:00 the day after. I guess a glitch in the generation, handing over or (most likely) processing of BT's "customer migrated" message.
My trigger for moving was a letter from BT saying that due to their price increase they would not charge me for ending my contract early if I contacted them on a 0800 number between 1 and 31 July. When I phoned they made a really handsome retention offer.
My only major criticism of PlusNet so far is that as a Business user I am supposed to get fast telephone responses round the clock. When I called the Business help desk at about 04:00 on the day after supposed activation I waited at least 10 minutes for the call to be answered by Residential, who told me that they could do very little for Business users and I should call between 08:00 and 20:00 when Business were working. I have since whinged about this and was assured that Something Will Be Done. It probably will.
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Adrian