Covid Requirements to Transit through Heathrow airport

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

I have to believe it is the most recent.

It says:

Transiting through England
There are different rules for Scotland, Wales and Northern Ireland.

Passing through England as part of an international journey is called ‘transiting’.

What you need to do when you transit through England depends on whether you transit airside or landside.

The rules are different if your final destination is within the UK, Ireland, Channel Islands or the Isle of Man.

Airside transit
Airside transit is when you do not pass through UK border control. You remain ‘airside’ before you leave the UK on your connecting journey.

You need to take a COVID-19 test before you travel to England.

You do not need to complete a passenger locator form before travel to England.

When you are in England you do not need to quarantine or take any COVID-19 travel tests.

Landside transit
Landside transit is when you pass through UK border control on arrival, but you leave the UK shortly after (usually within 24 hours). You may leave from the same airport, railway station or port where you arrived, or from a different one, so long as you travel directly to that port of departure.

You need to take a COVID-19 test before you travel to England.

You must complete a passenger locator form before you travel to England.

You need to do the following when you complete your passenger locator form.

select ‘Stay in the UK’ under the Your travel plans section
reply ‘I will be travelling for an exempt reason’ to the question about whether you are required to self-isolate on arrival
select the Exemption options, and then select ‘Transit Exemption’

Heathrow’s website corresponds with the U.K. government website stated above, which clearly states that you need a negative Covid test before flying. Vaccination has nothing to do with this requirement.

Of course, things may change by next week! I haven’t seen the U.K. government as panicked in the past 21 months as they have been this week about the Omicron variant and trying to get everybody boosted asap. It’s looking increasingly likely that a full lockdown will be reintroduced in January in the U.K..

You will need a negative covid test to board the airplane. Than, assuming you are transiting airside to change planes at Heathrow, that is all you need for Heathrow. You still need to follow whatever procedure and documents are required by Spain.