Some relevant quotations:
From CAP413:
Personnel providing an Air/Ground Communication Service must ensure that they do not pass a message which could be construed to be either an air traffic control instruction or an instruction issued by FISOs for specific situations.
An AGCS radio station operator is not necessarily able to view any part of the aerodrome or surrounding airspace. Traffic information provided by an AGCS radio station operator is therefore based primarily on reports made by other pilots.
NOTE: Air ground operators must not use the expression ‘at your discretion’ as this is associated with the service provided by FISOs and is likely to cause confusion to pilots
From CAP452:
Air Ground Communications Service (AGCS)...is not viewed by the UK as an Air Traffic Service because it does not include an alerting service as part of its content.
The CAA are not entirely blameless for the confusion caused. 'Duty of Care' has crept into CAP413 which these days includes examples of phraseology provided by AGCS operators which include provision of traffic information (It didn't used to). This alone muddies the waters and blurs the edges between AGCS, AFIS and ATC.