Expenses for nights on Agency work

Can I put a new twist to the discussion ? The agency I work for pay us travel expenses, the amount relates to how far the client is from where we live. £6 for local ones and £15 for those outside the area. Now this payment goes down on our pay slips as “travel” and doesn’t get taxed. The agency says that its a way to get us tax-free money.
2 questions, is the agency scamming me? Can I still claim mileage from HMRC?