Just to confirm Timatic still says the following for arriving from Spain:
Passengers entering or transiting through the United Kingdom must have a negative COVID-19 test result. The test must have been taken at most 3 days before departure of the last direct flight to the United Kingdom. Tests accepted are: antigen, LAMP, nucleic acid and PCR tests. The test result must be in English, French or Spanish.
Passengers details (name, date of birth or age) on the test result must match those stated in the passport or other travel document. The test result must also include the contact details of the issuing laboratory.
There is no reason to expect check in agents will be requiring a PCR only.