Information on the criteria used to identify gifted and talented students is available on the website - www.nagty.ac.uk
I think the schools tend to use CAT scores and similar - not sure if they use the actual 11 plus results. Clearly, looking at the website there are a number of different ways schools are able to use to identify G&T pupils.
The process in my daughter's school is as follows:
1. School write to parents saying that your child fulfils the evidence requirements laid down by NAGTY and ask for permission to submit the results to NAGTY.
2. NAGTY send an enrollment pack to child via the school.
3. Child is given interim membership on school's recommendation which is upgraded to full membership if the child confirms they wish to join.
It is possible for a parent to put their own child forward if they can provide some proof of eligiblity.
NAGTY membership is open to 11-19 year olds. Therefore it tends to be at secondary school level that children are registered. At primary level schools may themselves identify G&T children and there is usually an adviser who is supposed to help schools to nurture these children.
If the school has identified a child as gifted or talented I assume that this would be good information to submit in an appeal as evidence of the child's high academic ability.
There are lots of children who are NAGTY members at my daughter's school and I guess this would be so for all grammar schools.
Hope this is of some use.