If the car has had bodywork repairs, then the paint thickness will vary, as has been commented above, so if your paint is uniform thickness then that's fairly emphatic proof that it's never been repaired. "Faded paint" (although I've never seen a 4 yo car with faded factory finish paint) would be fair wear and tear, so they again would not have a leg to stand on/
It sounds like they're trying to scam you, or coerce you into paying extra. Name the leasing company, so we know who we should direct our future business to.. Or not.
*Edit: I've looked up Manheim. They specialise is vehicle "de-fleeting".. so my guess is they want to charge you £1100 on behalf of the leasing company to "fix" your car (wash it and give it a polish) so that the leasing company is so grateful to receive the car in such pristine condition, that they give Manheim yet more work...
It may be worth taking the car to a good body shop to get their opinion. If you're a girl they might do it for free.... but if they agree with you/think it's a load of cobblers and sound like they can articulate a professional opinion, then you might need them later to provide a report (which they might charge you for) if/when the invoice arrives and the arguments start. Otherwise if you're a member of the AA or RAC, they might also be able to advise and help.
Take lots of hi-res' photos of the car when it's dry (give it a clean first, so they can see it's immaculate)... and ideally on a sunny day, since that makes the paint look more uniform, so you can use them in correspondence with the scammers/leasing company (whoever sends you the bill).
Ralf S.