The most difficult part of the Passenger Locator Form, is the Booking reference for the tests you bought, I have had the problem that the form expects a certain string of letters & numbers & sometimes they just wont work, so you may need to fib, (even tho you ACTUALLY have bought them) no one of any note actually checks the forms other than the check in desk of the airline, Ive not had to show it again when entering the UK, & when you do the test, no one other than you gets notified, its a sham, just to try & make it unappetising to travel.
You just need a string like RANDX2017723 the numbers are made up, & are never checked. but if you dont get the form to accept it, you cant continue & complete the form, & then CAN'T get on the plane, even if you have all the receipts etc, without the completed form, you are staying in Portugal...