Hi I'm due to fly to th Uk from Malaga on 6th August then back to Malaga on the 18th August. Does anyone know if this is possible as I would only have quarantined for 12 days? Thanks

The legislation says you must quarantine for the length of time stipulated or the length of your stay if that is less than stipulated quarantine period.
Heres a screen shot of legislation for your circumstances

But who is going to know?

Most of your friends you infect in your local pub who know you've just come back from a high risk country.

Not if you´ve been isolating for twelve days

Then what is your question, "Who is going to know" about?
Know what?
Know that you are isolating? Pointless question then.
But it reads like who is going to know if you are not isolating. If so, then my answer stands.

In that case given the posters original question they have complied fully with the UK's legislation so your question who knows matters not. However you have to provide on entry details of self isolation address etc and I believe random but not routine checks can be carried out and I believe one has to accept that condition. The whole thing about quarantine self isolation whatever one calls it really relies upon common sense and people doing as advised/told as much as anything. However remember there is limited track and trace in the UK all be it that the infected person needs to know your details to pass on should the worst happen.

You could be making a trip to see if your eyesight is ok. Apparently that is acceptable.

With regards to OP, it is apparently acceptable to go on holiday outside the UK during your quarantine period, so in your case you would travel back to the UK, go to your place of quarantine for the 12 days then leave for the airport to go back to Spain, it's a strange quark of the rules.

Thank you for your replies

We have customers that fly every week, some every two weeks.

they are still doing it now.

not one has been contacted..

You are allowed to break quarantine to leave the UK.