Shopping was as much fun as anticipated.
Back to the point in question. The NATS licence includes an obligation for 'The making available, to users, of services other than Core Services, using the navigational infrastructure described in the AIP as at the date of the coming into effect of this Licence'.
Sadly, I don't think anyone actually tried to define what navigational infrastructure the non-Core Services users actually use. So it's up to NATS to make this judgement whenever they consider removing an en-route aid from service. You might also notice that, like many things in the Licence, it is not exactly set up to meet future changes - you might be able to rely on having whatever just happened to be in place in March 2001 but NATS appears to bew under no obligation to provide anything else that might be desirable in the future. I guess that will be down to the CAA or Government to provide....