advice please re changing ISP/phone

[ShirleyTemple]

We need to change from Demon which was good when it had tech support in the UK and is not really good anymore - mostly because of the diffuculty in understanding the very strong accents of the Indian staff but also we often get 'our computer is down at the moment' - which I think is code for 'I don't know the answer to your question'.

We have a charity website which we want to keep online..and also want to keep our phone number on our cable line - transferring it to the BT line before the change....but having been told that sometimes migrating can take 7-28 days our techie suggested
we ordered broadband from BT, then when it is installed we could cancel our cable line

BT will not instal a broadband connection unless we order another BT line at a cost of
£120 which our charity cannot afford.

This all sounds very complicated but what I really want is advice on how to retain our
Demon service (currently on BT line) until our new ISP is installed - was to have been
BT but now not so sure !

Also how can we retain our cable phone number and get it transferred to BT ?

Would any of the 'one month trial' (AOL?) or providers with just one month contracts be of any use ? Is there any way we can use our cable line with a new ISP and keep our
Demon connection with BT until the new broadband is installed ?