Okay, I will concede that LEAs can fund ABA programs for children under the age of three. I believe they SHOULD. Besides the ethical issues, it would save them a lot of money in the long run.
However, most LEAs I am aware of do not go out of their way to be agreeable to ABA programs for children under the age of 3. Especially if the diagnosis is "mild." To access any kind of ABA funding for a 2.5 year old, the parents will need to go to tribunal. I do not know of any LEA that will provide 30+ hours of EIBI as a matter of course for a child under the age of 4. I would LOVE to be proven wrong in this. Please do tell me if I am wrong. Maybe my experience in the UK isn't typical.
If a parent has access to ABA in the states and the child has mild autism, I bet that child is making a lot of really great progress. If this was my child, I would not want a substantial delay in treatment. The data are very clear that intensive intervention is important in the early years (The Early Start Denver Model data are so encouraging!). If the OP were to move to the UK, she will most likely need to self fund a program to avoid treatment delays. She will then need to deal with a tribunal, in a foreign country in order to access LEA funding.
I have been very harsh in my answer because in my experience, people in the USA can't believe how far behind the UK is in establishing ABA as a standard treatment. Their assumption is that the children will receive the same standard of care, and IME, that isn't the case. The standard of EIBI in America is often fantastic, especially when insurance is funding it.
I have seen the NHS refuse to accept a diagnosis of autism from abroad and insist on re-diagnosing. This can take months. This may not be standard procedure and only happen in my quirky neck of the woods. The definition of mild autism in the US may very well differ from mild autism in the UK. In the US, the rate of diagnosis of autism for children without a severe learning disability has increased almost 80% in the past 10 years. We have not seen a similar increase in the UK (at least in my patch). A child that young with mild autism may very well be diagnosed with "global developmental delay" in the UK because "it is just too soon to know."
My advice remains the same. I would consider delaying a move by a year or two.
I take moondog's point that some people in the NHS do provide behavioural based service. In my experience, these people are fantastic and they have taken to ABA of their own accord. EIBI (and this is what we're talking about here) is not a standard treatment offered by the NHS (but again, it should be).