I think that you will find the problem of running International (Europe or further afield) is simply one of frequency.
For some routes from Exeter (acting as a regional collector) one rotation a week would be all the pax that they could regularly get in one place at one time. The cost of setting up a route for once a week is not going to balance.
For the bizpax to be interested, for say Europe, you have to have a morning and evening rotation to make it possible to do a day trip. If you don't run six days a week, skipping Saturday, then you will not get biz pax in enough quantities.
Brymon, in their independent days, were perfect of course as were other small British operations. They could make their money feeding into LHR and folks could make the connection easily. Everyone was happy.
Then the politicians would not allow a third runway and slots became the currency of the last two decades. BA just kept on buying slots, sorry I mean competing air lines, as they could not buy slots directly. They did this last year with Manx. Shifted the routes to LGW - as they did with Brymon. They then get some long haul slots and immediately ensure that the connecting passengers who might have wanted to go on those flights - have to connect from LGW/LTN/STN/LCY and so start looking around!
Your only hope is that Bristol Lulsgate expands a little further, as you can, I think, connect to it from Cornwall?
I can but say, route the feeding flights to Northolt then you have a short ride on a comfy air-coditioned train to a choice of five terminals for your departures.
On this hobby-horse, does anyone know the sort of percentage split between UK and International at LHR? My thought is that N'holt becomes domestic only, with no customs facilities.