The logic behind the ACE base was purely down to aircraft performance. Even with the high MTOW 737-300's based at BLK & BFS, on days where headwinds are strong or the aircraft had a particularly heavy payload they wre tech stopping in FAO.
So to eliminate this either 737-800's had to be based at BLK & BFS or better still put one in ACE and boost capacity on other ACE-UK routes a la MAN & GLA.
With regards to the 'tech' issue - there are various standby aircraft in the UK, so worst case scenario it's gonna be a 4-5 hour delay whilst it is scrambled down there. No different to any single aircraft bases any other airline has.