I think it can depend on which SEN tbh, and what you mean by 'good'.
Some boroughs are so atrocious that you are more easily able to win funding for an independent school. Some can have a good reputation but still fall far short for the individual needs of YOUR child, even though they are doing well for others, thereby making it impossible for you to argue that you need something additional/different.
Many boroughs are adopting agressive strategies against provision now because if they become known for being SEN friendly, parents flock there and deplete their budget. Until there is a national strategy this will always be the case, so many that WERE good, are not so any more.
Westminster are probably the best, on account that the price of accomodation is such that parents CAN'T flock there.
Richmond currently are trying to bring their tribunal rate down so are listening to parents and trying to do deals with suppliers but who knows how long for.
Barnet have been easier to get provision from but are tightening things, although still not as hard as most.
I could probably list the bad ones more than the good tbh.