It is normally better off buying through a bank loan because the APR is normally much cheaper.
However, if you can find a decent deposit (usually 25-50%) then you can usually get a 0% APR for the term of the finance deal.
Look around and play the different dealerships against each other, they are all desparate for business right now and will cut each others throats to get your money.
It is advisible to check your credit history at experian.co.uk to make sure you have no red flags against you and so you can get mistakes fixed before applying for credit. They do a free 30 day trial. They offer a credit score for a fixed price but it is useless and is offered as a gimmick to get your money.
A note about UK credit for the americans here. There is no more universal credit score like there is in the USA, each lender has a list of perfect creditors. You may the chosen one for one lender but the devil to another. There is no blacklist. Each lender score you on many things at the time of your application, where one may reject you another lender may not.
Paying cards and loans off in full is not the best thing either and can actually get you rejected, meeting monthly payments without fail always looks better because it tells the lender they can make money off of you (interest is profit).