I've used a firm operating out of Stansted before now which were great and not that expensive, literally a one plane outfit. 'Course can't lay my hands on the name and number - if I track it down I'll get back to you.
You could try:
1st Charter based in Kent
01622 880188
Very swish - rock star stuff.
Also, why don't you ring Glasgow Airport Authority and see if they what small charter operators are based there.
Good luck!
CJ