How many shares in total have been issued? This will determine your percentage of the whole and thus the extent of the rights you have in law. You must understand your rights to help you, or an adviser, to begin to assess the value of your shares. For example, although a minority 26%+ has more rights than =<25%
Do you want to sell or retain? Usually in a small (apologies for the assumption) private company it makes little sense to retain once left working for the company as you have no control but can be a nuisance by exercising your rights (call/attend shareholder meetings etc) .
If you want to sell then best strategy is to convince the others you do not want to sell but sit back and enjoy dividends while holding directors to the duties they owe in law to yourself as a shareholder. This will raise the sale price.
But as others have said check carefully before you l;eave that you have not signed a Shareholders Agreement in the past which might require you to sell and possibly under a fixed method of valuation. Also need to check out the Articles of Association. You can save a lot of tax by reaching an agreement, depending on your reasons to leave (constructive dismissal triggered by conduct of others?) , with part of the monies paid being compensation for loss of office (tax free up to £30,000) rather than taxable share sale.
Call me next week if you would like further advice (no charge).