Just to add that the main piece of information they will need, if you are expecting a procedural service, will be your ETA at the beacon ('Cambridge Approach, G-ABCD with information November, 3000ft on 1021, estimating the CAM time 21, request a level for the hold.')
They will then probably ask if you want any holds - in case you want them for training - unless they are busy in which case they will issue a level and an expected approach time. If you have a pre-booked slot and you hit it reasonably accurately then you should be fine.
Having never really flown approaches in CAS I guess that feels normal to me!
Tim