I must admit that i've never worked anywhere that has given employees an allowance for working at home. The reason being that the additional costs of heating, lighting etc, are much less than the cost of commuting to work e.g. running a car, parking, fuel, being able to prepare food at home, compared to buying coffee/lunch near the office etc so the employees are nearly always much better off financially without any additional allowance.

Clearly your situation is different as it is a redundancy situation and you want people to accept working from home as an alternative to leaving and taking a redundancy payment.

I don't think you will find a 'market rate' for home working as it will be so rare so really it will come down to how much you think you need to pay to get people to accept the alternative way of working. I guess the decision to close the office is to save costs, so whatever you pay, you would need to be careful that it didn't end up costing more than keeping the office open.

Andreia
Preferential tax treatment of home working costs comes in two types.
Firstly, fixed allowances of up to £18 per month, where no supporting proof of costs is required.
Secondly, the reimbursement of additional costs that are specifically incurred to be able to work from home. Full receipts and proof that the additional cost is essential to be able to work from home. Any reimbursements of this kind will certainly be the subject of HMRC audit and checks. Examples could be the need for a printer, adding heating to an attic to use it exclusively for office purposes.