If you owe money on the mortgage (which I assume you do from your question) then - unless the other joint owner goes on paying it - the property will be repossessed. It will then be sold and what will happen depends on whether the sale price covers the mortgage (including arrears of interest up to sale date) and all the selling costs. If it does you have nothing to worry about because there is no debt. If it does not, then the mortgage lender will eventually try to recover the debt from either you or the other joint owner - you are both fully liable for the full amount and they can chase either of you, or both.
Going abroad is no guarantee that they (or their agent, or someone they sell the debt to) won't find you. If they do, what happens depends on the laws relating to this type of debt in the country you are in. In UK, they are civil matters and criminal authorities such as interpol are not involved, but I can't say what the position is abroad - it varies from country to country and I did hear about someone being thrown into prison in another country for a debt they left behind here.
In principle, it is never a wise idea to go away and leave this sort of problem behind - you may be lucky (so long as you don't return to UK) but it could easily come out the woodwork to hit you several years down the track.