Ouch! Plenty to go at there

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An O2 Access connection would not disconnect any more than the BT one does. You would actually be on the same connection from you to the exchange. All that happens is the exchange equipment then routes you to O2 servers not BT servers.
O2 does not have any traffic management or punitive throttling over 100GB like BT, but does get prolonged spells of congestion and I think not just in the evenings. So I don't recommend it.
I believe from what I read that Bulldog when it was independent and used C & W was quite good. It was when Pipex then Tiscali bought them they went badly downhill.
What I hear now re C & W is that vivaciti and Xilo are quite good on it though with occasional problems - the same as Demon who are shifting a lot of customers onto it but Demon with far higher numbers of course. With Xilo and vivaciti you do have the benefit of small-company support rather than useless call centres.
You could look at vivaciti's Surfwise 75 product. That is resold Murphy I think, not Entanet, so is still your BT type connection. It is a 75GB FUP not fixed limit and is claimed to be flexible.
BT certainly offer retention deals! There is a bit of a spat at the moment
going on here from someone who wants such as deal as a new customer. I think there have been some rather better deals mentioned recently than the ones he quotes.
A MAC is valid for 30 days from issue and should lapse if not used. If you use it then most ISPs take your notice period as starting from either the request or issue dates. I have a feeling Orange take notice as starting from the migration date, which has to be legally unsupportable. Some require you to give separate notice of leaving (maybe that is where the Orange customer went wrong), but you have to be very careful not to mention Cease or Cancel, as that is often taken to mean Cease/Cancel your ADSL and so you end up cut off, charged £23 for the cease, unable to get a MAC/migrate, and unable to order for a few days after cut-off while the BT databases catch up. Then a connection fee from your new ISP.
So far as I know BT are well-behaved in this respect. You would just ask for a MAC, then they may offer you a deal so you withdraw the request, or you reject the deal and get the MAC. What happens wrt the notice if you take a MAC should be in the Terms & Conditions, but it certainly will not be an automatic cut off.
I think before contacting BT you should decide where you would go if you didn't get a suitable deal, then ask for the MAC and see what happens. That way you also have something to judge the deal against, not just your current service.