Having3: My DS has just finished primary with a v similliar demographic - probably higher FSM.
The most important thing is that you visit the school - a proper visit, not what you pick up from the nursery drop off- and talk to the head and teachers, and observe classes. See how you like the way the school is run.
Our experience was this: The children with ESOL were given support, but would be in a separate space with the learning mentors for part of the time, and R an all the KS1 classes had a teacher and 2 TAs, so all the children were grouped according to their needs for any particular session and helped by TAs and the teacher. It was fine. Any children who arrived with little English were fluent in no time, and as Quint says, they do very well generally because the cognitive process needed to learn more than one language increases your skill and understanding of language overall.
As in many London state primaries, everyone in DS's class was in a minority of some kind - there were Asian, Black British, first generation french-speakiing black African, polish, portuguese, white Welsh, Irish, English, Eritrean, Somalli. When everyone is a minority, no-one is a minority. It has been a wholly positive (for non-white DS) experience.
Look at what the school do, atmosphere, quality of displays, different activities, how do they support high ability / lower ability etc etc.