Nobody here in the UK knows what the travel rules will be in September - or even next month!. You may be able to avoid quarantine arriving from Spain, you may not. And quarantine will probably be different for a non-UK resident who cannot just 'stay at home'. 'Vaccine Passports' - really just paper on on-phone certificates - exist and are being discussed with the EU (for UK & EU approved vaccines) but not yet agreed.

France has just imposed more restrictions on visitors from the UK so the rules can and do change weekly.