There is a register of MP's Financial Interests. Here, for example, is that for Rees-Mogg. There appears to be a minimum level before a shareholding or property interest has to be registered.
https://publications.parliament.uk/pa/cm/cmregmem/220711/rees-mogg_jacob.htm
Otherwise, as stated above, you can uses Companies House records, but of course you have to know the Company name. If you locate an appropriate Company, you can then look at the Company's website which
may give other holdings for a Director.