I work as a school with similar resourcing, pupils are meant to “have potential for graduated inclusion” and “specific not general learning difficulties” but I get sent severe and complex non verbal P3 children all the time.
It being the holidays is not an issue – the SEND office work through. You need to go through the plan and outline the outcomes you cannot provide for based on what the plan specifies. Then you should describe how the pupil’s education is not an efficient use of resources ie how they won’t be able to access mainstream like the others requiring additional staff or whatever and finally prime parents to go direct to send tribunal if they don’t get their first choice placement.
For the LA paying base funding to an enhanced resource and then paying a special school full funding is a waste of resources (and transport may represent an inefficient use of resources).