Can a contractor sign a contract and also witness the same contract?

Can a contractor sign a contract (in this case, to carry out building work), and also witness the same contract? This is in the UK.

Alasdair Taylor's answer to Can a contractor sign a contract and also witness the same contract? (631179625)

Where the signatures to a contract need to be witnessed, a party to the contract should not witness his or her own signature, nor the signature of any other party to the contract.

Of course, in many cases under English law a contract will not require signatures to be witnessed for the execution of the contract to be valid.