1. A solicitor may be able to advise on whether your ex can be made to go on paying. However, be careful - solicitors costs can be high & you will not necessarily get legal aid. Being on tax credits is not in itself a justification - it depends on your total income & assets.
2. If you have no objection to the house being repossessed you could simply inform the mortgage lender you were leaving (but make sure that, if you hand the keys back, you do so with a letter - keep a copy - & get a receipt for them) & stop paying.
3. They will eventually sell the house & the shortfall (i.e. the amount owed on the mortgage plus all the selling costs, less the sale price) will be a debt which is due from you & your ex on a "joint & several" basis. This means either one of you, or both of you, can be pursued for the debt. If you wanted to wipe out your liability for the debt you could consider bankruptcy or - if your total debts are below £15K & you satisfy some other conditions - a debt relief order. You could get info. on this from CCCS, but it is not an issue until you know how much the debt to the mortgage lender is.