MPs should not be allowed to hold shares in any businesses, and should not be allowed to hold either paid or voluntary positions in any private sector organisation.
Having any conflict of interest, real or perceived, declared or otherwise, can undermine the integrity of the political system and the public's trust in government. MPs are supposed to be the voice of their constituents, committed to acting in the public's best interests at all times. MPs are paid over £80k pa to do one of the most important and prestigious jobs in the country. This job should be an MP's sole professional focus, without any potential bias to support personal financial interests.