Question regarding transit through UK airport.

Yes its totally fine, 15 euro Antigen test in a local hospital in Spain or 30 euros in a clinic and you can use that to Transit airside or not (max 24 hours)

https://www.gov.uk/guidance/coronavirus-covid-19-safer-air-travel-guidance-for-passengers#transiting-through-england

Usually cost, countries with extra requirements for fully Vaccinated travellers are usally much cheaper (in my experience anyway)

I did a 30 euro Antigen spit test in Fuengirola and it cut my travel cost by 300 euros getting back to Germany (Restrictions were lifted for Spain so flights were totally full, as were any connecting flights on places where you could enter or transit without a test)

Basically whereever has the most hassle you can usually find really cheap flights.

Must have done around 40 or 50 tests in the past 8-9 months, it's not a big deal and at least then you'll know if you are contagious and can't fly.