It's not an employer-funded perk, though.
Companies don't do it for the goodness of their hearts, they do it becuase they save money as well. The employee saves on Income Tax, the employer saves on National Insurance. The saving more than covers the additional expense.
Except, of course, in the MOD where the work would fall on the existing staff and any NI savings would be recouped by the Treasury, so the don't bother - bad luck the members of the Armed Forces, you just lost your Income tax rebate.