What is the 'Deconfliction Service'? High Speed Flights
  
  2.36 - When receiving an ATS surveillance service, certain ex-military aircraft types have been granted a CAA exemption from the
  Air Navigation Order requirement to fly at an IAS less than 250 kt
  below Flight Level
  100. In order to alert the controller to this higher speed profile, pilots of exempted aircraft shall, on initial contact, prefix the
  aircraft callsign with ‘FASTJET’ or ‘FASTPROP’ (depending on
  propulsion type), e.g. “Kennington Radar, FASTJET G-ABCD request
  Deconfliction service”.
  
  Use of this prefix shall be confined to initial contact with ATC
  agencies for periods of flight during which operations at airspeeds in
  excess of 250 kt are intended. Once acknowledged, it will not normally
  be necessary for pilots to use the prefix in subsequent transmissions
  until making initial contact with other ATSUs.


Source: Manual of Radiotelephony, Edition 2016

What is meant by Deconfliction Service?
A deconfliction service is a specific type of air traffic service.

Normally, outside of controlled airspace, air traffic controllers do not alert pilots of potential encounters with other aircraft (conflicts).

However, there my be other services offered outside of controlled airspace, typically these are Flight Information Services (FIS).

In the UK, four air traffic services are distinguished outside controlled airspace: Basic Service, Traffic Service, Deconfliction Service and Procedural Service


  A Basic Service is an ATS provided for the purpose of giving 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.


source: UK Flight Information Services CAP 774, February 2015

Note the last sentence.

The deconfliction service is defined as:


  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 issues headings
  and/or levels aimed at achieving planned deconfliction minima, or for
  positioning and/ or sequencing. However, the avoidance of other
  traffic is ultimately the pilot’s responsibility


Again, note the last sentence.