The Post Office website states you need to "bring a pre-printed paying in slip to pay in cheques" and "a branded envelope for your cheque deposit":
https://www.postoffice.co.uk/everydaybanking
The Barclays website (as an example) says "To pay in cheques, you’ll need a pre-printed paying-in slip and a cheque-deposit envelope. Fill in all of the details and hand it to the Post Office staff. Cheques deposited this way will take two days to reach your account.
You’ll find pre-printed paying-in slips at the back of your cheque book. You can order them and the envelopes by calling us on 0800 169 3091 or by speaking to us in branch":
https://www.barclays.co.uk/ways-to-bank/post-office-banking/
So, as I read it, you DO need a pre-printed paying-in slip and YOU ALSO need a paying-in envelope supplied by your bank.