If your friend won't split the house (ie. buy you out or sell his share) then I think you could force a sale but it probably wouldn't be worth the hassle.
You might be able to get your current mortgage company to assign one or other of you the full existing mortgage without any further deposit (provided you, or your friend, earn enough to cover the new mortgage) and then it sounds like a straight transfer of the property would be appropriate which would mean whoever sells their share loses their share of the deposit.
If you really want to get out of the position then offer to give him your share and talk to your mortgage provider. That way they gt the best chance of getting their deposit back, although it might take a few years. If you can't get a transfer of mortgage and you really want out then you'll have to sell and accept the loss of deposit. Would an option be to let your room out to cover your half of the mortgage which would allow you to move and rent something elsewhere until prices pick up again ?