CAP 774, available on the CAA website, explains the different types of service available from ATS outside controlled airspace. It spells out which ATS units can offer which service and explains the differences between the different types of service.
[It is essential that you understand the types of service available, exactly what they mean and which of them you are entitled to request - otherwise you may come unstuck when ATC personnel ask you which service you require!].
A visit to an ATC Unit at a radar equipped airfield would be helpful.
H49