Avianca has nothing to do with lounges at MAD for your itinerary. The operator of your MAD-LHR does. BA or IB? IB metal will leave from T4 and BA from T4S.

If BA, you won't be able to get back landside if you clear security to access the lounge at T4S. If IB, you can go into T4 with your boarding pass, enter the lounge, spend some time there, then walk down to arrivals.

No idea if Avianca passengers can access the IB arrivals lounge, which isn't much at all, but does have showers...