Dear OP - I may be castigated for this - but why do you give yourself all the aggravation of doing a change of flight in MAD? If you live nearby LHR - I can understand that.Indeed some days the LGW flight goes out at at egregiously early times - and I hate that as well. However, the LGW flight is extremely convenient. I used it last Friday and I avoided all that perambulation at Barajas. It is immense and most of the Air Nostrum flights are from those K gates. It's miles to walk. Do you mind me asking? It may be useful for others.

edited to say that I think that the Lounges at Gatwick are streets ahead of LHR if that helps.