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Cashing a cheque on behalf of someone else
It's possible to cash cheques by post. When I did this, it involved filling out a "paying-in slip" (I had a book of these provided by the bank) and posting the cheque together with the slip to an address provided by the bank. You could also bring the paying-in slip and the cheque to a branch and deposit them there, and it wasn't necessary that you were the account holder, just that the details on the slip matched the account you were paying into. I Googled "paying-in slip" and found the instructions for HSBC as an example: Paying-In Slips. It explicitly mentions that you don't need to be the account holder to do this, and moreover there are even blank slips in the branch, which you just need to fill in with the correct account details. I think the procedure is much the same for other banks, but presumably you could check the relevant bank's website for specific guidance.