Just to chip in we got dx at 2.5 our son regressed so obvious dx. to be totally honest the dx did not bring much in the way of support - well in theory we got an autism outreach service and SALT but neither have been very good quality & termly at best. We see the paed every 4 months - now extended to 6 months but he just listens to us moan about the terrible services, he can't do anything. We don't get OT.
Once we realised how poor services were we started ABA. Initially we did this ourselves just paying for a supervisor which cost £400 a month and we did 20 hours a week. It was worth doing because it gave us a way of teaching him - he has moderate autism but bright. After about 6 months we were totally burnt out and employed a tutor for 5 hours. We also now have a volunteer who does 5 hours a week. Sometimes you can recruit volunteers from local universities.
We were able to show progress with ABA which the LA services were not showing and applied early on (before starting ABA) for a statement of special educational needs. LA have fought us all the way and we go to tribunal in Autumn. We will be broke by the time we get there but hope to win an ABA package funded by LA.
You can't get ABA without doing it and proving it works for your child.
Like Star it has been a horrific experience dealing with LA in fact more stressful than the dx.
You can get some extra funds by applying for everything you are entitled to including disability living allowance (you can get this without a dx but more likely with - Cerebra website has good guide to how to word things from ASD perspective). If you get higher (?middle) rate DLA you can apply for carers allowance if you earn less than £90 a week after tax, nat insurance and childcare costs irrespective of what your partner earns. You also get an extra amount of tax credits for disability - although how long this will last with new govt is hard to say.
PEACH, Autism Partnership and Treehouse websites are good places to start to find out about ABA.
AP have a social skills workshop for £15 coming up in June in London you might find interesting.
Surrey has an active National Autistic Society branch I think. It would be worth finding out about local pre schools / schools with a good reputation for ASD from your local branch / other parents. National NAS website has details of workshops and courses. Schools are usually asked to put in first 20 hours of support themselves from budget they are given but how this will work with proposals for new academies is unclear. Mostly support is given in terms of hours of 1:1 aide, they can be great or useless. Training in ASD for aide will probably be a days course if you are lucky. They will not be expert in teaching social skills etc. Again there should be some autism outreach support or similar who will advise school / nursery - quality varies.
I cannot honestly say my son has benefitted from services we have received mostly they have been useless but by trying and failing with them we will hopefully be able to show he needs funding for something more specialist.
What I would say is don't underestimate the amount of help even a child who is mildly affected will need. ASD is more subtle in girls at that age and I hear that they can be more social so disability can be more hidden; but it will become more apparent as she goes into school because girls are typically much more articulate, more emotional thinkers, play more imaginative games and their friendships more complex with boys (thats a terrible generalisation but you know what I mean). Often higher functioning children need more not less help as severely affected children won't need to grasp the intricate nature of playground games, teasing, friendships etc its actually those who are more able to access mainstream that need more help to fit in and perhaps are more aware that they are different.
Behavioural (ABA based) early intervention is proven to be effective especially for younger children so delaying intervention is not a good idea.
Other options are more play based developmental approaches eg Floortime - there is a lady in London who is Floortime trained here but there is little proven evidence compared to ABA and a developmental parent training research project (Pre school Autism Communication Trial) that just finished didn't show good results. But as a lower cost option it might be worth exploring. Sometimes you have to go and meet lots of people for free consultations to decide which way to go.
Overall we needed the diagnosis to get to tribunal. It is the first step in trying to get decent services. Without it getting support is even harder. In fact in my area you can't even get to see an autism teacher without a dx.
Your daughters interests sound identical to my son - although he is not social at all - he has the whole letter, shapes, memory, number thing going on. Its the more abstract parts of life he struggles with.