It's best to go on recommendations of others. They do not all seem to be equal. I've used RRG Macclesfield for my car and my previous one for 8 years prior, simply because they are passionate about the service they provide. There's a good smattering of petrolheads there which have over time become friends of mine. Sure it's a bit of a trip, but there's always a warm welcome so worth a drive.
There is a very good reason why RRG Macclesfileld did all the valve spring recalls on modified GT86's a few years ago. My (old) car being the first to be done in the UK.
What makes the difference is that there are a few dealers out there that do the mainstay of work on GR Yaris, so experience is always key here.
I've had friends from as far as Surrey come up to Macclesfield for their GT86. We even had the Duke of Marlborough up last week which is a fair old trek for him.