The tertiary volcanics were the last event in Scotland.
https://www.geolsoc.org.uk/Plate-Tectonics/Chap4-Plate-Tectonics-of-the-UK/Tertiary-Volcanic-Province

Interestingly the Older Granitic rocks are being studied for their hydrothermal potential, although the bulk of the data for geothermal gradient is derived from sedimentary oil exploration basins. For the depths required to gain the temperature benefit and necessary permeability to make any system viable it is likely fracking would be required. The temperature gradient is quite variable, I spent many years looking at the data from all over. I was on a borehole in the NW highlands where we encountered artesian water. The flow was strong enough to kick a couple of hundred feet of drillpipe out of the hole and the water was distinctly warm. Such events keep you quick on your feet as well.