This is a new one on me, but it looks as though the Admissions Code is on your side. It doesn't cover your exact situation, but I think there is enough there to allow your case to be considered under these rules. As ever, the words "must
" or "must not
" have legal force:
1.9 It is for admission authorities to formulate their admission arrangements, but they must not:
h) discriminate against or disadvantage disabled children, those with special educational needs, or those applying for admission outside their normal age group where an admission authority has agreed to this under paragraphs 2.17 to 2.17B;
Admission of children outside their normal age group
2.17 Parents may seek a place for their child outside of their normal age group, for example, if the child is gifted and talented or has experienced problems such as ill health. In addition, the parents of a summer born child may choose not to send that child to school until the September following their fifth birthday and may request that they are admitted out of their normal age group – to reception rather than year 1. Admission authorities must make clear in their admission arrangements the process for requesting admission out of the normal age group.
2.17A Admission authorities must make decisions on the basis of the circumstances of each case and in the best interests of the child concerned. This will include taking account of the parent’s views; information about the child’s academic, social and emotional development; where relevant, their medical history and the views of a medical professional; whether they have previously been educated out of their normal age group; and whether they may naturally have fallen into a lower age group if it were not for being born prematurely. They must also take into account the views of the head teacher of the school concerned. When informing a parent of their decision on the year group the child should be admitted to, the admission authority must set out clearly the reasons for their decision.
2.17B Where an admission authority agrees to a parent’s request for their child to be admitted out of their normal age group and, as a consequence of that decision, the child will be admitted to a relevant age group (i.e. the age group to which pupils are normally admitted to the school) the local authority and admission authority must process the application as part of the main admissions round, unless the parental request is made too late for this to be possible, and on the basis of their determined admission arrangements only, including the application of oversubscription criteria where applicable. They must not give the application lower priority on the basis that the child is being admitted out of their normal age group. Parents have a statutory right to appeal against the refusal of a place at a school for which they have applied. This right does not apply if they are offered a place at the school but it is not in their preferred age group.
I'm not sure that this bit applies strictly in your case, but it is possibly a little more grist to the mill:
Children from overseas
2.19 Admission authorities must treat applications for children coming from overseas in accordance with European Union law or Home Office rules for non-European Economic Area nationals. Non–statutory guidance on this is available on the website of the Department for Education.