Don’t see why Bisley would be any different to anywhere else?
A range is a range.
As for the temperature of the day - the heat haze will be worst when the temperature difference is bigger and humidity is high. So likely to be at its worst on cool damp day in Scotland…
Ultimately, when you’re shooting big strings, your mod is going to get hot. A few microns of fabric is not going do much. The differences between a £200 bespoke tacticool cover, some vet wrap or an old sock are going to be a minimal.
Until someone makes water cooled mod covers, the choice is not about functionality, it’s about looks.