Hi Gareth,
I don't think this is going to work as every crawler by Google is run by the same useragent: Googlebot. What you could do but what I really wouldn't recommend to do is generating the robots.txt automatically. Check if the IP address of the user is in another country and then Disallow them. It probably won't work as the crawler from let's say Germany could also be used for the UK.
Also the specific data for a countries search engine gets collected first and then gets looked at to see what they need users to serve, not the other way around that content gets acquired for a specific country.
Hope this helps!