Try buying a stock trials bike it is a mountian bike modified for trials just raise the seat up and you have your trail bike, minus the 2 extra gears in the front. There are lots of stock trials bikes in the uk for a good price , try
http://www.cleanbikes.co.uk
http://www.bikes4trials.com/
You can also get a diamondback mod or stock trials off of www.ebay.co.uk, never tried them being uk only, but was told they take one hell of a beating for the pound\dollar\euro whatever eh?