Early years support was provided by the local authority. You need a referral. Ask the team who gave the diagnosis to refer you.
This is just my experience. I can only tell you how it has been for us. Diagnosis was quite straightforward. My eldest was clearly autistic from birth and had his formal diagnosis of autism by the time he was 2 and a half. It was actually his early years teacher who flagged our youngest up. He presented very differently to our eldest. He got his dx at 3.
You need to know the law because you will be fighting the lea for appropriate provision and may well be fighting the school too. Then college. Then fighting for placements and adult support. And for direct payments. You need to know the Sen code of practice inside out and upside down. My experience is that you'll be told this doesn't exist, that doesn't exist and only when you can prove it does will it magically appear! You'll be told they can't meet your child's needs because you have to think of the other children too. They'll gaslight the fuck out of you. If they don't have the funding for a particular need, your child doesn't have that need. 
Imagine going to the hospital with your bone sticking out of your leg and they say oh, your leg isn't broken. We don't have any casts or anyone who can set it. It is fine. You don't need it.
Re picking your battles - I think all parents should tbh but when your children have autism you have to have a different approach. What things actually matter? Health and safety is number 1 of course.
Food battles on the other hand are not worth having. Eye contact - forget about it. It can be added in later. Going places because you think the child should have this or that experience - is that because the child actually wants to or because you have a picture in your mind about how childhood should go? What you think should be fun for a young child can actually be torture for a child with autism. Sensory overload is hugely stressful.
My eldest - 22.
He was in nappies until he was 6. Until he was I'd say about 10 he had soiling issues. He didn't really talk until he was 7 and then he talked like a toddler. He was in senior school by the time he was talking without any issues. Before he could talk he was very aggressive. He bit and kicked and he even broke my nose. He wouldn't sleep, he would be locked in repetitive actions and transitioning him from one activity to another was hell. We were told that he would never talk and would need to be in a special school. We were really put under pressure to teach him makaton and use pecs.
We felt that wasn't the right path for him and were accused of being in denial.
He is now as I say, 22. He is in his second year of uni, he has a driving license and a car and is in the process of buying a house. (We're not wealthy, he has a trust fund). He won't be able to live independently without any support. He has support and adaptations/accomodations at uni and he will need a support worker to visit him as well as support from us when he moves into his own home.
My youngest is 21. He was DX at 3 and far later had a dx of ADHD. He was extremely passive as a toddler and we later realised that was the ADHD passive innatentive. He stopped wearing nappies at 5. He never gave any indication that he even realised when he was wet or dirty. He went to bed one night in his nappy, I changed him in the morning. He took off his clean nappy, threw it across the room and went straight to using the loo. He talked but only delayed echolalia for years. He has an amazing vocabulary now. He can talk but he isn't interested in having a conversation. He will give you the information he wants to give you and he isn't interested in your reply. He has PBS now (positive behaviour support) which is going really great.
He is aggressive and can be violent. We have locks on our bedroom doors for safety. He has assaulted many of his 1:1 and needs 2 people with him when he is stressed. At school he had 1:1 normally, 2:1 in stressful situations and they also carried walkie talkies to call for back up. He was left without a placement in college after he attacked his 1:1. There's loads more but this is going to be longer than war and bloody peace.
He is at heart a lovely, kind, sweet and funny young man (as is my eldest) but he cannot filter out any sensory input and the world is hell on earth for him.
Imagine you are in a crowded room full of people yelling and sirens going off. The people all have horrible BO. There are flashing bright lights everywhere and you are wearing clothes made of wire wool. And someone is shouting over the noise and demanding that you carry out a task.
Could you cope? Because I couldn't. Would you lash out? Because I'd be trying to punch my way out of that!
He will never be able to live independently. We will keep him here with us until we can no longer care for him. During that time we are preparing for a transition to a group home.
But again, this is just my two. Every child is different.