I sounds Adam (knowing that you are a pilot from your postings on here) like you would have been better off perhaps doing a bit of research on here and dealing with it yourself!
I mean I haven't actually chartered a private plane before but as a PPL holder have used Signature etc a lot at various large airports with friends and family and would know exactly what questions to ask to ensure I got what I wanted if I ever did.
Perhaps a reason to use a handling agent in the main terminal (Menzies? - Not familiar with Luton) was to save cost? Could they have been cheaper than Signature/Harrods?