There is no defined maximum contribution an employer can pay with UK legislation. However, the employer may need to consider more than just the ability to fund unlimited amounts.
Any contribution made over the members available annual allowance will create an annual allowance charge which will subject the member to a tax charge. This charge will be levied on the member at the highest rate(s) of income tax the member pays. This can cause potential issues for the member as the contribution that has been paid is a legitimate contribution and cannot be refunded, so the member will have to find the money for the charge. Legislation is in force to allow, in many circumstances, for the pension scheme to pay any tax liability on behalf of the member through a process called scheme pays. This is available to both personal and employer contributions.
For corporation tax purposes the employer may wish to ensure the contribution is ‘wholly and exclusively’ for the purposes of the business (i.e., relevant to the position held within the company).