to me it seems fairly straightforward:
You need a PCR test to get on the plane that is taken within 72 hours of the flight departing
You need a PCR/antigen/lateral flow test to get in the ground that is taken within 48 hours of the game (you initial PCR test can be used if taken within 48 hours of the game)
You need a PCR/antigen/lateral flow test that is taken within 72 hours of the flight departing to get on the return plane (your PCR/antigen/lateral flow test used for stadium entry can again be used for this)
You need a 'day 2' PCR test upon arrival back into the UK and this must have been booked by the time you land back in the UK or you'll be fined
from all of that, i think you can get a PCR 'fit to fly' test thursday with same day results (DAM health) which will cover you for all but the day 2 test. if you're leaving any sooner or returning any later you'll need a lateral flow test while in porto which i believe are available for about £30 from a number of places