Ok, I'll give you three recommendations that fit within your price range.
Mac:
Option 1:
13-inch MacBook Air (Standard Config) £999.00
Option 2: 13-inch
MacBook Pro with Retina display (Standard Config) £999.00
The big differences between these two are hard drive space, laptop size, and speed. The Macbook Air has more storage and is very thin/ light, the Macbook Pro has a faster CPU and more RAM for faster processing and is a bit heftier.
Windows
Option 3:
Microsoft Surface Pro 4 (256GB / Intel Core i5) £1,079.00 *This is just about to be released, might need to wait a month before ordering*
The Surface Pro is quite different than the Macs, as it is technically a tablet/ laptop hybrid. It has a touchscreen and stylus available that allows for drawing on the screen, which might be a nice feature for developing plays and showing movement on a page. It does come with a keyboard as well though, so you don't have to fuss with the on-screen keyboard nonsense of most tablets.
Of the three I would personally choose the Surface Pro 4, as the on-screen manipulation could be very helpful in a dynamic setting like coaching.