In the UK, do we need to ask potential LPs to self certify as a High Net Worth or Sophisticated Investor when we share a PACT? I.e Do we need to incorporate an investor self declaration as part of the PACT?

The PACT is a non-binding letter of intent and therefore you will not need to ask LPs to certify. That being said, you should avoid LPs that are not wealthy and meet the basic accreditation standards. Once you're ready to start a closing process, your lawyer will draft the proper declarations in the official legal docs.