Car Allowance And Millage

Hi, I currently receive a car allowance from my company and I also claim mileage (0.25p) for journeys committed, The car is owned by myself and does not belong to the business I am responsible for making sure I have MOT Insurance TAX etc. My employer has now said that they are dropping my mileage in line with advisory fuel rates. When I asked the question to my employer if this applies to me they said because they pay a Car allowance it does apply and I now can only claim for 0.15P in line with the advisory fuel rate Is this correct it doesn't feel right? Granted they are paying partly to the running of my car but the money they pay dose not cover all the running costs. This is the link they sent out to us: https://www.gov.uk/guidance/advisory-fuel-rates

Hi,
Advisory Fuel rates are for company cars however it is between you and your employer how much they pay for business mileage in your own car.
If they pay less than the approved rates, you can make a claim to HMRC's Personal Tax Department for tax relief on the difference,
please see the following on gov.uk:- Claim tax relief for your job expenses
Thank you.