Hi Miss Roadie:
My DDs are at a 'good' school (as rated by OFSTED) with a large number of EAL students and about 10% FSM (not particularly large and genuinely lovely kids). The question isn't what additional language - but what is their general attitude toward learning/ being at school.
At our school, genuinely, I can confirm that nearly all the parents of EAL students (Korean, Indian, Sri Lankan, Pakistani, Ugandan, Kenyan, Syrian, Iranian, Lithuanian, Polish, Chinese, etc....) value education and they're the kind of parents that really push their children to do homework, a bit extra (many go to their national language schools at weekends and I know several do extra maths/ English because parents are not happy with standard of work at our school) and many families prioritise work on English Skills. Often when the parents themselves don't have great English, their children do - because they are relied upon so heavily to read letters, speak in shops, etc....
What I will say is any time anything goes wrong with SATs or OFSTED, our school Management Team/ HT or the Governors have a tendency to announce that it is because of all the EAL students and/ or high mobility. The latter is simply untrue - certainly in the two year groups my DD's have been through maybe 7-8 pupils have left over the course of 4-5 years, but the new pupils have all stayed. For the last 4 years the council has publically reported a 0% mobility rate in Y5/ Y6 for the school.
The pupils leaving were often here because there parents were studying at the local University. One Korean firm decided to close operations here, so several families returned to Korea - their children were all fantastic students. They're often replaced by pupils from very similar backgrounds - although often different countries and/or local parents who failed to get into the school and wanted to (often for reasons of convenient after school care - the other local school doesn't offer much or many after school clubs - which is a factor and certainly after school care was the deciding issue for us in choosing this school).
I suspect when you really look at the data - which as a parent I'm not allowed to for data protection reasons - you'd find the problem lies elsewhere. Unfortunately with a single form school - presenting that kind of data publically would more or less indicate who didn't do well, so I can understand why they don't.
Genuinely, Miss Roadie - EAL isn't a huge issue - how the children are is. Are they eager to learn, is the school a bright and cheery place, do the children seem happy and engaged with learning, do parents sending children to the school speak well of it, etc.... These really are the questions to ask. If you aren't certain your DC will be a high achiever - look at the data for KS2 and see how the school does with low and average ability pupils (that's now published - so if you see only 13% of low ability pupils manage to make the government target of NC Level 4 in English/ Maths - that should be the worry, not their EAL status). You should also look at how many pupils achieve NC Level 5. This isn't snobbery - this is about understanding whether the school is capable of producing pupils/ supporting them to this standard.
Frankly the world, and given you're in London, is becoming a very blended, multicultural place - it is an advantage to your child to already have grown up in that kind of environment and be comfortable in it.