If the dealer is backtracking on the promised service history then you will have strong grounds to reject the car for a full refund, as per the 2015 Consumer Rights Act: www.honestjohn.co.uk/how-to-reject-a-car-your-cons.../

If you want to keep the car then you could try and negotiate a discount. But the missing service history will affect the car's long term value. Depending on the make and model, this will range from 10% to 20%.

I would demand a 20% refund from the dealer if I was looking to keep it, as the first thing you'll need to do is pay for the car to be serviced and inspected by another garage.