Applications usually go in early in the calendar year, however you can do mid year applications. The best thing is to contact the schools that you are interested in and enquire about availability of year 1 places.
With regard to delaying entry for your July born - it is not impossible and worth enquiring. There have been reports about a UK mother winning a court case recently because her council rejected her application to delay. There has also been this recent article in the Daily Mail:
http://www.dailymail.co.uk/news/article-3059609/Parents-summer-born-babies-right-delay-entry-school-new-Tory-plans.html
I am the mother of an August born, who started school a week after turning 4, saying that she coped very well, but the first year for me was very stressful!!!!
Personally I think if you can get back for the start of the school year - end of Aug, beginning of Sept (dependant on your county) then the children will get a full UK school year in.
I wish you luck - schools for me are probably the most stressful thing when I am moving - we put ourselves under so much pressure to get it right first time, but you will get a better picture of things when you are there.