Some EU block exemption rules were retained by the UK after Brexit, one of which was that a manufacturer cannot void your new car warranty if it is serviced at a non-franchised garage. However, it must be serviced in accordance with the manufacturer's service schedule and specifications including parts and service items used, and a suitable record of the servicing must be maintained. We would also suggest checking the terms and conditions of your lease, as these may specify that servicing must be carried out by a franchised dealer - as the vehicle is being leased it remains the property of the finance company at all times, so they may make specific stipulations in order to ensure it retains its value over the period of the lease.