I still think building a rifle turret (while it sounds awesome) is a terrible idea; but for anyone wishing to investigate further Michael Reeves crazy experiments are quite a good place to start.
https://www.youtube.com/watch?v=Q8zC3-ZQFJI
You could likely use Tensorflow to have the turret track rats it sees too.
The only big issue would be reloadingshort of sacrificing something like a Steyr H5A the best option would likely be a 10 shot mag and a nice beefy servo to cycle the bolt.
It's completely do-able especially if you have a light rifle like a contour you can use.
Not sure anyone with an FAC should risk it though. Fun topic for an inspection. "What's that?"... "Oh that's nothing to worry about officer, it's just my Rat hunting turret"
There are quite a few things the UK government have not banned because they have not needed to, such as a gatling airgun; while it may be legal you can bet once one is built it wont be for long.