Some general advice when changing broadband suppliers.
To change your broadband supplier you will need to ask your existing supplier for a MAC Code which will need to be provided to your intended supplier for them to be allowed to replace your service. Some suppliers are unscrupulous and will refuse to give you one - however if you threaten to report them to OFCOM they will succumb as they are bound to by law. They are not however bound by law if your account is not in order.
The usual case when requesting the MAC code is for the customer service teams to enquire why you are moving providers and then possibly offer to match/improve their offering. It is a sad state of affairs but it is usual with broadband suppliers (like with mobile phones) for you to left on a crap tariff until you ask for a better one.
When you look at moving broadband suppliers you need to look at the contetion ratio that the provider has. Demon offers quite a low contention ratio which is why they are rated highly. The contention ratio is how many other subscribers are using the same resources at the telephone exchange - if the contention ratio is 50:1 then if 25 users are high bandwidth users then the service will be very slow as your all competing for the same resources. The less people to compete with then theoretically the better the service (depends on individuals usage patterns).
I would suggest rather than move poviders straight off the bat - speak with demon and tell them what you can get and see if they can better your deal - you may be surprised to find you get alot better deal for no extra outlay and no loss of service.
Cheers