Can a PDI Offer Motorway Lessons?

It depends entirely on who's in the car. A PDI cannot take a provisional licence holder on the motorway, full stop. A PDI can take a full licence holder on the motorway.

Provisional licence holders: ADI only

Taking a learner driver on the motorway requires a fully qualified ADI, in a dual-control car. It's not just a guideline, it's genuinely an offence for a trainee instructor to conduct a motorway lesson with a provisional licence holder. This applies regardless of how close a PDI is to qualifying. The actual regulation is here: The Motorways Traffic (England and Wales) (Amendment) Regulations 2018 (legislation.gov.uk)

Full licence holders: a PDI can

Once someone already holds a full licence, a PDI can take them on the motorway, for refresher or confidence-building lessons. The restriction is specifically about provisional licence holders, not about the road itself.