I have found that there is often some 'Service Drift' with the type of service you receive, but don't count on it.
When flying in the Midlands, maybe tracking towards a VOR, I have been given Traffic information even though I only asked for a Basic service.
As I understand it although there is no legal requirement for an ATC unit to provide traffic information when under a Basic service the rules which
military units operate under require them, wherever possible, to assist pilots to maintain safety. Of course legally speaking Basic service means no traffic information and that is part of the "contract" between the pilot and the controller and pilots should always assume this is the case. However locally we often work an RAF unit for basic service and they usually provide useful traffic information for which one is most grateful.
What really peeves me though is when pilots ask for Traffic service on a gin clear day thereby increasing controller workload and the service they can provide to other traffic.
I also understand that when the AEF are flying with cadets on board they are now required to request a traffic service - I guess this is a symptom of the blame culture after the odd collision when cadets have been on board.