I have done Labour already, so here are the rest.
Lib Dems (page 137 in full doc)
Strengthen worker participation in decision-making, including staff representation on remuneration committees, and the right for employees who collectively own 5% of a company to be represented on the board.
SNP (page 17)
We support increased employee representation on company boards, believing that this can help bolster long-term decision-making and improve industrial relations. We will urge the UK government to consult on how best to deliver such an increase.
Plaid Cymru (page 11)
Supervisory boards of major companies who employ more than 500 staff will include elected employees, to ensure scrutiny of the management in the interests of the company as a whole, building on established practices in industrially successful countries such as Germany.
Amend company law to ensure that medium and large companies have employee and consumer representatives on their boards.