Without wishing to sound anal financial advice in the UK is regulated and therefore no one unless qualified can give you firm specific advice on here.
Generally though.
It is better to have no loans than a loan and savings.
The dealer may be able to get a lower apr than you can a loan. However this is because the loan is secured against the car (and they get a commission).
In the same way Mr. Car Salesman says beware of the banks advertising rate also be careful of the dealers headline rate as these are often variable as well depending on the model/optional extras/your deposit/mileage allowance. Additionally you could be tied to the dealer network for servicing - there are anti competion laws against this for your warranty though these don't apply to PCP and some HP because the car belongs to the finance company not you so they can stipulate who maintains the car.
A cheeky way to do get the dealers finance incentives is to put the biggest deposit down possible and then pay the finance off early, some agreements allow this without penalities if done say within 14 days. Your dealer won't be happy though as they tend to loose their commission for the finance.
Tongue in cheek comment - If the dealers were competitive they would employ customer service staff and not sales peeps
