I don't know how much you know about how support/funding works here in the UK. Apologies if you already know all this. A good place to start is here:
www.ipsea.org.uk/Pages/Category/get-support
This outlines how schools and local authorities should support Dc with additional needs.
I'm afraid to say that all boroughs in London (and all councils across the UK) are experiencing very difficult times with regards to funding. The number of Dc with additional needs - and with EHCPs - is rising quite dramatically. This is putting councils and health authorities under a lot of pressure.
I know of Dc who are on two-year waits for speech and language for example and of course COVID hasn't helped.
I live in London and I know many parents who have Dc with additional needs and unfortunately I've not heard anyone sing the praises of any borough.
Are there any areas of London you've looked at yet? Is there anywhere you need to be in terms of work/family support? Have you looked at any schools for your Ds? Also do you know what your budget is?
What will help you is to know SEN law - that way you will be in a better position to ensure your Ds accesses support, where ever you live.
You may get more detailed responses if you can pick out some areas in London that you're considering. Unfortunately we're all looking for the holy grail of a council that offers 1:1 support and easy funding!