How should I use credit card while not affecting my credit file

hellok

Hi all, I know this question might be a bit silly, but just hope you can shed some light on.  

Background:
4 years UK address history, got my mortgage last year, been having barclay card (£500 limit) for roughly 1.5 years. 

I am buying a car worths £1k, as AmEx is offering 5% cashback up to £100 for the first 3 months, I am planning to get (switch to) the everyday cashback card (since Barclay has no cashback reward at all)

Question: 
I once have paid roughly £200 three months in a row (£500 limit on CC). Ended up one of the credit reports has dropped slightly, as I have used increased limit. 
Assume I am given an Amex card, if I pay £1k in one go, is it very likely my credit score will be impacted?

Thank you 

D3xt3r5L4b

Does the dealer accept Amex? Or any form of credit card?

Have you spoken to Barclaycard about changing your card to the Reward card that offers the cashback as well?

Pay no attention to your score or rating. No lender ever sees it. 

MinuteNoodles

You DON'T have a uniform credit rating

There's no such thing as a credit blacklist. This is a myth. In the UK, there's no uniform credit rating or score, and there's no blacklist of banned people.

Each lender scores you differently and secretly.

This means just because one lender has rejected you, it doesn't automatically mean others will. Though after a rejection, it's always important to check your credit file for errors before applying again.

Of course, if you've got a poor credit history, or had problems, it can feel like you're blacklisted. Credit scoring is intuitive – would you lend to someone with a history of not repaying? However, on occasion there are firms that specialise in lending to those who have had past problems – though they then charge a whacking rate.

The tools that lenders use to decide aren't universal either. As well as your credit file, they also look at application information and any past dealings they've had with you, and use the three sources of information to build up a picture of you.

bradders1983

The above isnt quite true. American Express certanly keep lists.

SpreadableToast

bradders1983 said:

The above isnt quite true. American Express certanly keep lists.

I think they're referring to some sort of universal blacklist, rather than the blacklists that pretty much every lender will have for bad customers.

OP, aside from the dealer possibly not taking Amex, and if they do quite possibly not allowing you to put more than a small deposit on a credit card, Amex are particularly picky with who they take on as customers.  With a fairly limited history on a very low limit card I wouldn't fancy your chances in being accepted.  I suppose it doesn't hurt to try but I wouldn't get your hopes up.