Similarly, if you want to go Luton to Northolt don't file for FL100. If you want to stay inside controlled airspace, and I understand you prefer to, expect whichever SDR goes to BNN and 4000ft.
If you are happy to go below CAS it will either be an SDR to BPK or BNN (depending on Northolt's runway) and 2400ft.
Here to help.
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vintage ATCO
Air Traffic Services to the landed gentry