According to G&T co-ordinator dh, in his school: theoretically, the teachers nominate the students that they think are gifted in their subjects, but there can't be more than 10% in any one subject, and the students have to be gifted in either English, Maths or Science plus one other subject at least; and talented is performance and art - so music, drama, sport etc any one aspect. You don't have to be both G and T.
Plus they cross-reference with the CAT scores - 128 or more in any battery is the 5th percentile, or 120 or more in two batteries.
They don't use SATs levels.
But there's nothing clear cut, and it should be something like 5-10% of the school population is registered as G&T, regardless of whether the school is super-selective or underperfoming. So a school could have a huge number of students that meet the CAT scores criteria, but they can only register the top 5-10%. Alternatively, they might have no students that meet the requirements, but they still have to register 5-10%.
Having a gifted and talented register of 5-10% of total school population is a legal requirement for secondary schools.
However, that was before change of government, so who knows how long it will last.