FWIW it was me that posted the original comment about using the Glasgow ILS for cloud break to Cumbernauld.
I've done it several times and the original advice to do it came from Cumbernauld airport.
As has been posted already, you just ask the controller you are already talking to and they will arrange it for you quite routinely.
On one occasion, I tried to do it and couldn't break cloud high enough to be safe and ended up actually landing at Glasgow. Both landing fees and handling costs were reduced by 50% under the Strasser scheme but still hurt! Both ATC and the handling agent were very good though.