As the free TMC info is no longer available, can the units in the car be 'upgraded' to receive TMC via subscription?
I'm not sure what you mean. I have a feeling you are confusing trafficmaster and TMC.
Trafficmaster units that displayed the information in the highline IPKs are slowly being switched off. I've not had an activation anywhere local for weeks. There's a little box behind the IPK (Instrument pack (IPK from the German)) that recives the signals from roadside transmitters, and as the transimitters are being switched off, they don't work. There's no real option to do anything about this!
TMC on the other hand is broadcast over FM radio. As far as I know, Classic FM is still the only broadcaster in the UK that carries this though. It only works on 75s / ZTs if you have satnav. The radio receives the signals and works out if any traffic problems are on your route, then gives you the option to reroute if it is. You have to have a satnav CD that supports this (the TMC location codes are on the CD/DVD) so the subscription is by buying the latest CD/DVD.
Does this cover what you mean?