99% of companies do not have written shareholders agreements, and rely on the "default" shareholders agreement under Table A of the 1963 Companies Act. Under Table A, his shares will automatically vest in the Official Assignee, who, as Steve has pointed out, will try and sell them. I have seen some "cleverly" worded shareholder agreements which state that the shares must be transferred for their nominal value to the bankrupt's co-shareholders in the event of a bankruptcy
In practice, it is very difficult for the Official Assignee to sell shares in private companies, as the Board of Directors can simply refuse to register any transfer of shares, on the basis that the company is "private".
A 50% shareholding can create issues, as there can be deadlock etc. It stands to reason that the outcome will depend on the size, profitability etc of the company.
Once he is bankrupt, you will have to locate another director to maintain the statutory minimum of 2 directors.
If he is owed directors loans from the company, then the Official Assignee will insist on payment.
Given the legalities and uncertainties in dealing with the Official Assignee, it might be wise to buy his shares now. If you buy them now, you must pay full market value for them. (If you do not buy them now, you might have the Official Assignee as your co-shareholder for the next 25 years!)
Jim Stafford