Hi Emma,
Some people shear in July because of their area, Scotland is later then Devon due to the fact that in the high hills of Scotland there can still be a keen frost in may whereas Devon warms up earlier.
Also there are only a few shearers shearing the whole of the British flock so it is inevitable that some people get left until later in the season, I shear some sheep in august and then again in April as the fleece if left to grow for a full year gets too much for the sheep.
Hope this answers your question,
Thanks,
Philip