There is absolutely no way any car dealership has sales staff who aren't on commission.
Firstly, a list of what NOT to do.
Don't say you don't know anything about cars.
Don't say you want to pay cash.
Don't agree to any requests to "come into the office and talk about it".
Don’t pay any attention to the useless as fuck "Seventeen thousand point AA inspection" bollocks that they will tell you about. It means fuck all. There are people out there who genuinely believe a man from the AA is going around inspecting cars for sale in dealerships and signing them off. In reality it just means a staff member from the dealership has attended a hald day training course with the AA and is "certified" to carry out some checks. It's bollocks, those checks never happen, it's just advertising.
What you SHOULD do.
Keep silent most of the time. If your buying outright with cash you won't get a better deal. Those days are over. DO NOT say something stupid like "what's your best price for cash".
Dealers don't price their own card, its done through a software package that checks the Internet for all car prices and sets one automatically. The market today means you won't get any discount off the list price, so don't bother asking. The only exception might be an old diesel, but otherwise there are no discounts.
You will get seven day drive away insurance, so don't worry about that.
Yes, you can drive away the same day.
If you're doing HP, please DO NOT focus on the monthly payment, it's the total price you must concentrate on. The monthly payment figure is how they hide the real cost of the car.
Work out what you want to pay, not what you want to pay per month. And stick to it.
If its a new car, you'll have a warranty and good protection. An out of warranty one will have a Mickey Mouse three month warranty, forget that, it won't cover anything.
Buy from a reputable dealer. Those car supermarkets are not great, you might find a good car, but be wary of upselling.
Do not buy their extra insurance, no paint protection, alloy wheel care package, upholstery cover, gap insurance. None of it.
If you want gap insurance you can get it online for a lot less.
Download an app like "Car Check", they are free and will give you a full history of any car you look at, all previous MOT reports and stuff. You will be able to get an idea of how the car has been looked after. Numerous failed MOT's means it's probably a lemon and best avoided.