Share options can be a true benefit particularly where you were granted them in the early days. The big disadvantage of them, as I think you have worked out, is that the exercise of options to get the shares will trigger a tax liability of 52% of the difference between what you pay for the option and what the share is worth (at date of exercise). If there is no market in the shares (i.e meaning that the company isn't quoted) then exercise may create a tax liability with no way of paying it. The questions I would ask are:
1. is the company quoted on a stock market. If so I don't see why you can't exercise the options, sell the shares to pay your tax and keep the balance. You can walk away into the sunset.
2. do the options lapse if you leave the company? Look at the option grant, or other documentation, to see if there are good and bad leaver provisions. If so say what these say about people who leave the company in relation to the options (i.e. do they lapse or get forfeited). If the documentation says nothing about such leaver provisions then you may be able to leave without having the options lapse.
3. While I appreciate that the options may have vested can they be exercised? In many circumstances options may vest but can only be exercised in a 'liquidity' event. This means that the options can only be exercised when either quoted or when the company is being sold.