I know someone who works for such a company but sadly they are based in Manchester.....let me know if you get really stuck and I may be able to pick her brains.
TBH I would think any well recommended electrician should be able to do the job for you.....unless we are talking football pitch dimensions of course!
We are just using local electrician to do our arena - he's just done our yard and done a cracking job.
Be aware though that if you have your stables/yard/arena on a private property the installation classes as domestic so electrician must be Part P registered
Alos be aware that if you are floodlighting you will need Planning Permission
