Which en-route services are available to UK GA pilots?
According to UK AIP ENR 1.1 "General Rules", chapter 2 "UK Flight Information Services" the UK FIRs meet the ICAO requirements with the following Flight Information Services (FIS): Basic Service, Traffic Service, Deconfliction Service and Procedural Service.

This is an extract of the contents of the chapter:


  Basic Service
  Provides advice and information useful for the safe and efficient conduct of flights. This may include weather
  information, changes of serviceability of facilities, conditions at aerodromes, general airspace activity information, and any other
  information likely to affect safety. The avoidance of other traffic is solely the pilot's responsibility.  
  
  Basic Service is available under IFR outside controlled airspace in any meteorological conditions, or under VFR. A pilot may change
  heading, route, or level without advising the controller. A controller will not issue specific heading instructions; however, generic
  navigational assistance may be provided on request.
  
  Traffic Service
  Traffic Service is a surveillance based ATS, where in addition to the provisions of a Basic Service, the controller provides specific
  surveillance derived traffic information to assist the pilot in avoiding other traffic. The avoidance of other traffic is solely the pilot's
  responsibility.
  Traffic Service is available under IFR outside controlled airspace in any meteorological conditions, or under VFR.
  
  Deconfliction Service
  A Deconfliction Service is a surveillance based ATS where, in addition to the provisions of a Basic Service, the controller provides
  specific surveillance derived traffic information and deconfliction advice.
  A Deconfliction Service shall only be provided to flights under IFR in Class G Airspace, irrespective of meteorological conditions.[...] A controller will provide traffic information, accompanied with a heading and/or level aimed at achieving a planned deconfliction minima.
  
  Procedural Service
  A Procedural Service is a non surveillance ATS where, in addition to the provisions of a Basic Service, the controller provides
  instructions, which if complied with, shall achieve deconfliction minima against other aircraft participating in the Procedural
  Service. Neither traffic information nor deconfliction advice can be passed with respect to unknown traffic. A Procedural Service shall only be provided to flights under IFR, irrespective of meteorological conditions. [...] the pilot remains wholly responsible for terrain clearance at all times. [...] A controller will provide deconfliction instructions by allocating levels, radials, tracks, routes, time restrictions, approach clearances and holding instructions, or use pilot position reports, aimed at achieving a planned deconfliction minima.