From what I hear the UK is quite expensive for keeping horses. The figures offered on here may not be in your currency, and don't really reflect the cost of a horse where you live.
Like one of the above posters say they spend $3 on a bale of hay (could have read that wrong) where as here in Australia (at least my part) you're looking at around $15 per bale, a bit less if you buy bulk. Their bag of feed costs them $12 where as mine is over $20, and I pay $40 for the farrier to trim (not shoe) my horse.
As you can see just those basic costs are markedly different. I recommend finding out where you would keep your horse and ask them their prices, and what it includes. I know there are places in my area with paddocks for $30 a week, and others have stables for $120 per week (not including feed). So look around and see what the costs are. Have a look at your local feed store and see how much your chosen feed will cost, as well as hay. Find out what the going rate for a farrier in your area is, some charge over $100 per visit, others much less.