I guess you was given the forms you got when you order the car, setting out the amounts and on the rear is the terms and conditions and in there should be a section on cancelling. mostly it’s part of the comsumer credit act and each maker has slightly different terms and a time scale should be clearly set out. They may of also sent you emails like honda did due to covid i could not sign stuff so had to be done electronically. Sometime consumer law is on ourside and is your friend you should get the full 3k back. The only reason i can see it being an issue is if you do it after they have registered the car to Mb.