Can you find out how long it will take to complete the process where you are? It may be worth hanging on for a diagnosis before making any decisions if it's going to happen soon (once we were referred for that final assessment stage it took us about 7-8 months to come to the top of the list where we are I think).
ABA is a controversial one, and it does seem like it takes a huge amount out of all your days (4 hours driving and 3 hours ABA, so practically a full time job), so I can see why you are weighing up the elements.
Before making a radical decision on moving I'd do some more research on how supportive any destination borough is more generally (e.g. how good is the education system for kids on the spectrum? What sort of support is provided)? From what I've read I don't think anywhere will pay for ABA, so the question would be what other services (SLT, OT, physio, etc.) you may be able to access in different areas, with and without a diagnosis. For what it is worth we are in Hackney and have found access to services really good, but we haven't needed a huge amount so we may not be a good test case.
Your DS is going to be in school for a lot longer than he'll be pre-school, so I would focus on good school support in any move (as you say, how easy it will be to get an EHCP and whether the LA regularly appeals all EHCPs would seem a key decision making factor - if you can avoid having that battle on your hands it frees up so much mental energy for more constructive things).
How are you finding the ABA so far? Do you like their approach? If you lived closer to it would you be happy with it? Are you both working, or would you consider delivering the therapy yourself with distance support? This is what we do - talking to an ABA advisor every 2 weeks by Skype but to the extent that we do ABA we "deliver" it ourselves: in our case it means we end up not doing much, but I am much more comfortable with it, as I know that I can focus just on things I am convinced will genuinely help my DD, and that I can make the judgement on what things to target and whether she is enjoying it or not rather than leaving that to somebody else. I didn't want to risk that somebody else would be discouraging stimming or pushing her beyond her boundaries or making her feel it's not OK to be herself, for example.
I think in the UK the authorities do not really believe in ABA, and I am not that convinced of the evidence for it (in particular on the long-term impact on mental health) - but on the other hand there doesn't seem to be any other support on developing self-care, emotional regulation and social skills, and I think some of the methods (e.g. breaking things down into small steps and giving positive reinforcements when your child achieves something new, ideas for ways to make things more engaging/fun) are genuinely really helpful just as parenting/teaching methods. Our ABA therapist has also given us some great ideas (like doing a diary to talk about good and bad things from the day and drawing pictures of things she achieved or things that upset her to help her work through them) that I'm not sure are really ABA at all, but I think have been really good for DD.
It's such a hard judgement call and it's such early days for these "therapies" that the jury is really out, so we are left as parents to just make our own judgements with no expertise which is a very hard place to be...