You are correct, for aircraft which are outside CAS over the whole of the UK in fact, however a pre note to Swanwick Mil is required (which can be done via various methods). As per the Pennine task, the AIP again states that the service is subject to workload so is not guaranteed. A NOTAM explaining the reduction in service and detailing affected areas would be helpful.
Apart from company reasons, there is no forced increase on route mileage on operations outside CAS when Swanwick Mil are not available. It's up to the operator to assess what service is available and make their risk management decisions. As a Swanwick Mil service is never guaranteed, the operators and pilots should have a plan B up their sleeve in case they can't get an appropriate service outside CAS. I suspect the high level of availability from Swanwick Mil leads many to dispense with this part of the flight planning process !!
The scenario does lead nicely in to an age old Conundrum, which is prevalent in the London FIR. When the TRAs are not active and Class C airspace exists above FL195, why don't Swanwick Civil provide ATC outside the published route structure ? Who is responsible for the provision of ATC in that CAS ? Someone has to be.