I wonder if you confused them by saying you had a "2 day test" but hadn't booked an official 2-day test. The official ones are sent off for genetic sequencing (or whatever it's called) if positive so the UK can keep track of variants.

It might have been better just to tick the "exempt from testing" option on the PLF and told the check-in staff that no test was needed. Your PCR to use on your return is of no interest to them.