Bonington boxes are really good and are in leicestershire but it depends what you are looking for. They made our last box which was lovely so would really reccomend them
I have a JSW horsebox, it is really well made, and horses travel really well in it. They are based in Northallerton in North Yorkshire, so may be too far away, but definatly worth considering. We were considering an Oakley, but we actually decided the build quality, especially in the horse area was better in the JSW, plus it was about £20K cheaper!!!!!!!!!!!!