shares in a horse

Have a look in Australia.  Unless you have money to burn, racing a horse in England will just burn a big hole in your pocket.
Unless of course you wish to spend 20,000 a year on a horse running around for 5k pound prize races a few times a year?

Very serious by the way, not an us vs them.
I bought into a horse, 5% share, 5 grand.  Had 5 races so far, including running for 3.5 million, and also scratched from a 2 million dollar race.

If you buy one from say the Magic Millions sales in the Gold Coast, they have their own races run specifically for horses bought at their auction.  The 2yo main race had a purse of around 2 million, as I stated.  That is on top of the main races like the 3.5 million dollar Golden Slipper (the richest 2yo race in the world).

Very easy to get involved with and the syndications are obviously targetting these races with their purchases.
As I said on a similar thread, my horse has already run for more than half of what Frankel raced for over his whole career, and mine is just a horse, that has had 5 starts.

England really doesn't have the same structure, and is really about the big stables in control of everything.  They are racing to create stallions for the world, and as such their prize money really is ridiculous. 
As far as bang for buck, you really can't compare the two.  Obviously though, you would struggle to see your horse running live.  But then, for 20,000 pounds a year, watching your horse run around to try make back 10% of what you are spending on it would make me cry more than anything.
I bought into a horse and 6 months later got to travel half way across the country to watch it run in the richest 2yo race in the world.  Pulled up saw and pulled out of the race lol, but still, a big thrill.  When you are talking about that sort of money, can easily jump on a plane to watch the big races.  Even if you only place, you are making more money (or spending less), still watching your horse run, and travelling the world as well..
And if you happen to get into a horse that doesn't do that well, chances are they are still running around in Sydney for 85k (AUD) a race anyway.

The costs are going to be the same.  But the prize money in England isn't worth the effort of putting it in your pocket.

Id find a small yard ........near you.....with a trainer starting out & take half share with him/her..........then in their interest for it to do well...........plenty of good horses in fields as it costs to get em race fit & find out............we had 4 & came out winning.......they say thats 66/1........a dream BUT a spare money game..................G.L.

Get Conor Dore to buy you a cheap one from a seller. He's an excellent judge and will get you to the winners enclosure without haviing to wait a long time. It'll be lower end but cheap and satisfying. Once you've got your win, sell up and try another one. You don't have to invest pots or stay in for too long if unlikely to be a return.

who are the trainers you have found that you describe as 'back street-ish'

it wouldn't surprise me if there are decent trainers in there

trainers who advertise horses for sale are usually trying to get rid of the dross left over that they bought at the sales.

they have already sold the better horses to their best owners and are now trying to recoup some of their original outlay on these second raters.
add in the fact that they will be trying to sell them for 3 or 4 times what they cost originally to guarantee them a profit and you will see that it might not be a great deal for you.

best to get along to the sales, preferably with someone who knows a little about horses (maybe even your new trainer or his rep), and try and pick up a bargain for a few grand.
after that it's all down to luck as to how good your new purchase is.

there are plenty of decent trainers up north who will do their best with your horse.

good luck