WH
The simple answer, is that you must adhere to the joining procedures at Denham (1000 feet inside the LCTR zone)
I can understand the Luton controller reminding you of the MSA - especially if the weather is a bit claggy - but it is up to you to follow the rules.
We get PFL requests a lot on LHR SVFR and we allow up to 2400 in the Denham overhead if co-ordinated with Northolt Apr.
Sorry if this is not the answer you wanted..
Luckily, there wasn't a Northolt departure climbing northbound off runway 25!! That would have increased your workload!