You/preschool do not need to wait for the community paed assessment before requesting an EHCNA. You can do that yourself now. IPSEA and SOSSEN are charities who have lots of helpful information on their websites. They have model letters you can use to request an EHCNA yourself.
However, even if you make the request now, you will not have an EHCP by the normal admission deadline, so you should make a normal application, anyway. If an EHCP is later finalised, the placement named in there will override the offer made via the normal admission process.
Unfortunately, many have to appeal, some more than once, so you may not have an EHCP by the start of the 26/27 academic year.
Without a finalised EHCP, the normal oversubscription criteria will apply. You will need to look at the school’s admissions arrangements to see the school’s oversubscription criteria, because not all schools have the same categories. Within the oversubscription criteria, some schools have an exceptional medical or social needs category. If the school has such a category, in order to be considered under it, you will need professional evidence proving, in the professional's opinion, only that school is suitable. The bar for this is high. In the vast, vast majority of cases, suspected ASD will not reach the threshold. A referral letter to community paeds is not going to reach that threshold. A letter from the preschool is also highly unlikely to be sufficient.
Contact the schools and ask to speak to the SENCOs.
Questions I would ask a mainstream school:
-How many DC with EHCPs do they have on roll?
-Class sizes?
-In any schools with smaller class sizes, do they have plans to mix year groups?
-What additional support with transition do they offer for those with SEN?
-What support do they offer for those who struggle to make friends?
-What support do they provide other DC with similar needs to your DS?
-How does the school reduce/prevent sensory overwhelm?
-How do they deal with dysregulation?
-Do they have a quieter room/lunch club DC who are overwhelmed with the hall/playground can go?
-Do they have a sensory room?
-Do they run a nurture group?
-What interventions do they run?
-What is their outdoor provision like?
-Where do children take movement breaks?
-Do they have DC who have reasonable adjustments to the uniform policy? (This is a reasonable adjustment, but some school are more accommodating than other and it will give you an idea of how supportive they are.)
-Do they have DC who can’t cope with the hustle and bustle of arriving/leaving school and have different arrangements for arriving at school (such as arriving 5/10 mins late via reception)/leaving (such as leaving 5 mins early)? (This is a reasonable adjustment, but you would be surprised how many schools are not supportive.)
-Do they allow DC to use noise cancelling headphones/ear defenders? (This is a reasonable adjustment, but again you would be surprised how many are reluctant to allow it.)
-If it matters for DS, is the site secure?
-If DS isn’t toilet trained, what facilities do they have?
If you want, you could ask about transition to Y1 - some schools have a more formal Y1 than others. For some DC with SEN, transition to Y1 is difficult. (Conversely some like the more structured nature.)