Hi
As others have said a desktop quote will be back with you in around 7 to 14 days, these are an over-estimate it seems with actual costs typically being much less. If you are running a business from that address you may qualify for a the government voucher scheme that will cover up to £2500 of the installation costs.
Also note that as mentioned, if you went ahead with a full survey to get a confirmed price, you may have to wait several weeks or a month or more before that order gets to BT for a surveyor, and then several weeks for an appointment, after that it can take many more months before installation is started and completed, some people its taken 9 months or more. You'll probably not be able to even place the order for a surveyor until you have completed and moved in, as the surveyor will usually want to enter the property. You might be able to get the present owner to start the process, however whilst they might be able to get the desktop quote, I don't think they could book the surveyor as it will all be in their name, and when you move in you'll probably need to start again.
With Cerberus, I know they will give you a desktop quote without worrying about if you are at the property, just use your name and the property address you are moving to. They don't post anything to the address, everything is done online, and you are under no obligation to continue, of course if you want the full survey, that's when you have to start signing things and their is a credit check that takes place, so you really need to live there.
Either way, you will likely move into the property waiting many months for a decent broadband connection, that's assuming FTTP on Demand isn't priced too high for you, so if you need to run a business and are reliant on the internet, you will certainly need some sort of plan for the interim period, which is easily 6 months or more.
Regards
Phil