There is no set trigger for an airport to install radar - it's up to the airport to decide if it wants it. It's normally when there's so much traffic about that aircraft using the airport will suffer delays. By co-incidence this is about the traffic level that people begin to think it's too busy to be safe without radar.
LARS is part paid for by CAA - although I think the whole business of LARS is under review at the moment - but it nowhere near covers the cost to an airport of providing the service.
Sadly, in the commmercial world that we live in, airports see ATC as the way to get their traffic in and out efficiently. If providing a LARS costs money and doesn't help the number one task then it's going to take a lot to persuade them to provide the service. (I would just point out that in my experience many controllers do not see things in that way and still think they're there to provide services to aircraft in their area - when they're allowed to that is).
You ask whether people think there's a need for LARS 'up there'? Well, it's not so busy up there so traffic avoidance is less of a concern and if you have an emergency you can always call Scottish FIS ....... or did I hear somewhere that the FIS was often withdrawn due to staff shortages?