I am puzzled as to how Brize can effectively unilateraly withdraw their service.
LARS is provided when controller workload permits. This means that you don't have a God-given right to the service. If you operate outside controlled airspace, either by choice, or because of licence considerations, then this is the way the game is played.
Even those operating under IFR in the airways system are frequently delayed by slots, in order to ensure that controller capacity is not stretched beyond already ambitious limits.
Take a look at what the AIP says about LARS:
http://www.ais.org.uk/aes/pubs/aip/pdf/enr/2010603.PDF
and note section 5.1 about service availability.
Hope this helps