Mine is in FS2, i.e going to year 1 in September so he's one year younger than yours. I have been to two parents meetings in the past year and was told that the children would not be assessed until they finish FS2. Compare to the children in China, they're far behind.
At this age, they are not really learning much, just playing all the time. Chinese children tend to do better than British children anyway.
I don't know how much you know about the school culture in the U.K but the teachers don't put any pressure on the children or parents either. Everyone is good!
All in all, don't worry about it. Hope you enjoy your stay in the U.K.
