Hi - I've never sent any of my cockers away to be trained but what I have done is attend a couple of 3 day residential training courses (which focus on the trainer - more than the dog) with Sunstar Gundogs - they are based in Dorset so not sure how that works for you. (Hope I'm allowed to mention these guys)
My youngest cocker is now 14 months old and I was hoping to go with her in September - but with the arrival of our new clumber pup in July - the timings just aren't right. I have however booked to go in April with her, and a friend with her dog pup of a similar age. We will both have had our youngsters out for the odd drive come Jan 2010 so will have an idea of what problems we have and something to work on while on the course.
From a different point of view good friends of ours in Wiltshire take in gundogs for training - what they say is that they usually leave a new dog to settle in for a couple of weeks before they start any training, you can imagine the "shock" to the system of being kennelled if they aren't used to it etc etc. He likes to get to know the dog prior to starting anything. You need to factor this into costing out the whole boarding school scenario. Also I know that Peter likes the owners to come back every, say 3 weeks, to see how each dog is getting on - he is also rather honest if he thinks the dog is not ready for training or isn't going to progress any further. I would imagine some "trainers" would just keep taking your money rather than giving you honest feedback?!
Hope it works out for you both!