Assuming the same standards are applied, an ATO can hold both UK CAA approvals and EASA approvals post Brexit, the only additional cost being the extra approval fees paid to one extra authority. In other words we can anticipate no or minimal extra organisational or admin costs, just the annual fees. I doubt that would create an issue unless EASA try to impose the costs that they did before in the example of the US ATOs. If that's the case UK ATOs can come to an agreement with an existing EASA ATO to operate as a satellite. I'm anticipating the IR schools in particular will take this path. I can't recall what the fee structure is for their direct EASA approvals post Brexit. My memory is not a great deal in relative terms