IS a "holding deposit" illegal?

General Discussion

My agent has found another tenant for my property, tenancy to start in one months time. Prospective tenant is now "uncontactable" as he is away on holiday.
He has apparently paid for his credit checks etc., which I suppose infers he is keen to take on the property.
However I asked the agent if the tenant would put down a deposit as a gesture of goodwill - to be deducted from his rent as and when.
The agent has informed me this is an illegal practice.
So I could wait for a month, then the tenant decides not to proceed - another month wasted. I have told the agent to keep the property on the market, just in case. He is reluctant to do this.
Any comments, please?