OceanSky and Harrods have Bombardier trained engineers so you'll be fine wherever you go in Luton. Signature is right next door to Gulfstream but I think they have some Bombardier engineers too.
Can let you have some contact details of the guys at all the handlers so you can compare if you like.
Finally, if your passengers like nice new lounges then Harrods is the one to go to.
OB