Covid test for return to U.K., time validity for antigen test.

The certificate needs to be 72 hours or less, but this is for when you arrive in the UK, so you need to allow the approx 30 hours sailing time. 

You may be safer arranging a test in Bilbao area. 
One in Santander is https://www.laboratoriodraconchadelafuente.com/

They speak English and have good reports.

The Brittany ferries website states this so it’s not from what I read arrival in U.K. it is departure from port of embarkation.

“Everyone entering England from overseas (including UK nationals and residents) must provide proof of a negative COVID-19 test taken within 3 days of departure to England.”

Anymore ideas on this please

This is incorrect - the test must have been completed within 72 hours of departure from Spain (not arrival to the UK).

Ok very confusing. Is it an antigen test or a pcr test or eirher taken 3 days prior to returning to UK. Alot of conflicting answers here.

P & O told me it's the time I will be boarding at Calais, not arriving in the UK. Still an awful lot of confusion around!

There should be no confusion, apart from the incorrect advice from 'wain654'.Please see link to gov.uk:https://www.gov.uk/guidance/travel-to-england-from-another-country-during-coronavirus-covid-19The key part is copied below...

1. Take a COVID-19 test in the country you’re in. To board your transport to England, you need proof of a negative result from a test taken in the 3 days before the service departs. (my comment - this is an antigen test and should cost about €30; this is vary clear that this is '3 days before the service departs' - in the posters cases, the date/time you leave Bilbao or Calais)
   

2. Complete a passenger locator form. You can do this any time in the 48 hours before you arrive in England.

In addition, assuming you are double-vaccinated, you also need to pre-book a Day 2 test package to be delivered to your address in the UK. You need the booking reference from this to add onto your passenger locator form. If you don't have this, you will probably be refused boarding, or at least refused entry by Border Control when you land in the UK.

By the time you're leaving it may well be the case that you won't need to be tested, in view of the announcement earlier today.

We have just done this journey last week, the antigen test 3 days before you leave Spain is fine, BUT you cannot complete the Passenger Locator Form until 48 hours before you are due to ARRIVE in the U.K.  So we had to complete the form at 9.15am on Thursday morning, drive to Bilbao, stay overnight in a hotel and board the ferry on Friday morning to arrive at 9.15 in Portsmouth on Saturday morning. Hope this helps.  Incidentally, we had to show all paperwork at Bilbao port, but U.K. border agency just wanted to see our passports.