Immigrationboards.com

Hi,

I have now both passports and I'm going abroud for first time.

I think the theory is the next:

-Use British passport to enter and exit from the UK.
-Use Spanish passport to enter and exit from Spain.
-Use any to enter and exit from other countries.

That's correct, right?

But which passport should I use when checking for my flight online?

For the flight to Spain. Should I use the British one or Spanish?

I think the company communicates this information to the UK border, so I am unsure...

Many thanks!

Your take on which passport to use when is correct.
As far as the airline is concerned they want to see you have a valid travel document for the country you are visiting. The data is tranmitted to UKVI.

Show both passports at check-in. The UK passport confirms your status in the UK. Your Spanish passport confirms your status in Spain. Show the appropriate passport at Immigration, i.e. UK passport at UK immigration, Spanish passport at Spanish Immigration.