The answer can found in CAP413 Chapter 3 as below. This has been a requirement since 2006. For those ATSU's using electronic flight strips this is of extreme benefit in order to immediately know what is required. When the ATSU replies thay may ask you to pass your message or they may only reply with their callsign which is the invitation to pass your detals.
By not including a request you increase the number of transmissions which can be very irritating in a busy traffic situations.
If you don't include a a type of service you will probably be given one such as Basic or Traffic (ATSOCAS) whilst outside controlled airspace.
Initial Call – VFR Flight
3.28 Normally, the initial call to an ATS unit should only include the minimum
information needed to establish:
1. The service that an enroute flight requires; or
2. The clearance/information that a joining or departing flight requires.
There is a very good aide memoire card from airspace safetyfor the Initial call, Pass your Details and Position Reports.GOOGLE 'R/T - Prepare your transmissions'