As the Racheee said, for state schools your child won't officially start until they are nearly 4. That said you are in Northwest London, so it is a unique place. Lovely place....just full of overachieving parents
Your friends are probably right, if you wanted to send your child to a private pre-school when they are 2-2.5 then yes, expect to have them on a waiting list by the time they are 1. It really depends on what school you want and how popular it is. I used to live near Hampstead and I know some of the pre-schools run out of certain synagogues are very popular around that area and are highly sought after. So I would expect the same from faith based private pre-schools too.
It all depends on what you are planning to do with your child. Don't feel like you have to do exactly what everyone else is doing. First step would be to research the local state nursery schools. If you are happy with their reputations and are happy waiting till your child is 3.5-4 until they go to school, then you really don't have to do anything for a while. If you are likely to want a few mornings a week to yourself and want your child in a social setting when he is a bit younger, start looking when in a few months.
Good luck.