complained to Luton
Why? surely they have the same right as you in be in Class G airspace.
Why are they not above 5,500ft and in the controlled airspace?
The jet could have been on it's way back from an air test to the north or out of Cambridge. The a/c can't simply join controlled airspace at any level due to other traffic that may already be there.
It may have been more expeditious for the jet to join Luton at 4A so as not to get in the mix of Essex traffic.
The default clearance for aircraft coming out of Cambridge that want to join controlled airspace to the south is to fly towards Barkway at 2.4A and contact Essex Radar. Essex will then when able give that aircraft further climb and routing in instructions when it's safe to do so.