I believe either should work. The Scottish website has an option for indicating you arrived at LHR or a few other English airports. The PLF is UK wide so should accept an English or Scottish code. I had a 4 hour layover before Edinburgh so I did the English system to save money. NHS Scotland sent me a couple of emails but it all seems fine. No one at the UK border will even look at the PLF only the airline before you board.

the emails for what it’s worth basically say we know you are coming to Scotland and you must follow the rules. They give the links to the Scottish system website for both Scotland and England arrivals. It’s the same address but the Scottish one ends in ?country=scotland