Always a pleasure to help with any questions related to UK procedures etc... especially considering the numerous differences between us, ICAO and the US.
Your starting to appeal to the investigative side of my brain now..lol...but I like a challenge.
Luton is already inside controlled airspace H24 so you cant be told to "remain clear of CAS," if your already inside as this would be a contradiction, just like when I tell my dog to get in his bed whilst pushing him away from it
Unless of cuorse the controller was having a bad 'air day (pun intended)
My suspision would be that, speaking hypothetically, you would have departed an aerodrome outside of CAS, lets say Biggin Hill for example. The approach/departure controller calls London for an airways joining clearance, however, due to the complexity of the surrounding airspace they cannot provide a joining clearance prior to your departure becuase then London would have to protect the level that you would be climbing to. Which isnt usually possible in the UK for various reasons.
So instead of being "cleared to join CAS on-track Ockham at 4000' squawk ****", London would give something like "cleared on-track Ockham at 2500' squawk ****, remain outside controlled airspace."
By stopping you off a 2500' that keeps you below CAS, until the controller is able to climb you into CAS.
Hope this helps.