Have you run this past your FEO?
I ask as a lot just love the worn Minimum and the .243 in their eyes is IT!
So I would suggest you speak to the Licensing Office or FEO and see what they say. it has already been mentioned what your use for the rifle will primarily be. Some Forces won't allow larger than .243 for Foxing whilst others are happy to allow larger calibres to be used. Unless you already have the open slot on your FAC then you might save a lot of time and hassle to see what they will
"Allow".
May I also suggest you try if possible to shoot a few different rifles and ideally ones in different chamberings and calibres and see what you like. There are those to whom the 270 Winchester is a brute
. It's also wise to see what your usual shooting emporium carries ammunition for JIC.