Would it be possible to commute from abroad (Madrid for instance) as long haul pilot?
Undoubtably! There are alot of BA and Iberia flights from Madrid! (Beware that the Iberia flights and the BA flights depart from different terminals but quite close to each other scheduled departure wise! So getting from one to the other is difficult).
You would be wise to seek out some reliable overnight accomodation in the London area though as, being junior, you might well get some of the earlier LH departures thus need to be in London the day before. There is, however, a very active accomodation board for casual bookings in the Crew report centre (CRC).
Edited to add:
There are some of the new DEP joiners to the 744 who are commuting from Spain, Germany and the Netherlands who are at the bottom of the seniority list commuting BUT I seem to remember that Staff Travel only kicks in after 6 months so you might need to factor that into you decisions. Many just rent a short term flat for six months and then commute. I depends upon your home/family situation.