I live in Watford. Pretty much all schools are over subscribed here. You have missed the admissions closing date for Reception (which was back in January), and as you need a Y2 place and a Reception place, you need to start ringing round schools to see which might have spaces in both years. Schools like Nascot Wood, which are very good and therefore very popular, will have a long waiting list already of people who already live very near but didn't get in. I suggest you call Herts Council School Admissions to find out how you do this without an address in the UK yet.
Here are the stats on admissions in 2011. You can see how close you have to live to each school - look at Rule 5, which is the rule you would probably be applying under (assuming your DC have no SEN or other medical needs, which would bump them up the priority list?).
This helps you find your nearest school (once you have a UK address) and the precise distance which Herts Council would use to measure it.
This is the guidance on In Year Admissions (which is you).
If your son is Y2 age, he will have to go into Y2. I don't know of any way in which you can change this, although probably worth starting a whole separate thread just on this - there are various experts on school admissions on these boards who will be able to advise you (look out for the posters called "phr47bridge", "admission" and "panelmember" in particular).
Re the commute - you need to work out how long it will take you to get to the specific part of London you / your DP will work in. I live in Watford, and the train is only 20 mins from Watford Junction to London Euston, but my whole commute takes (at least) an hour, because I have 2 tube journeys to take once in London, plus a 10 min walk.
Some good news is that Watford is pretty diverse - my DD is at primary in Watford and there is a significant minority of Pakistani, Eastern European and other ethnic minorities and nationalities.