"1) Why doesn't SEN money only go to kids with SEN?"
Gifted and talented is a form of SEN.
SEN stands for special educational need. The "S" is for special not stupdid. Its for any child who has needs not easily accomodiated in the classroom. Gifted children do have special educational needs and if the school feels they need extra one to one then the money to pay for it comes out of the SEN pot.
"2) "school action plus", what is that?"
Its when the school gets advice from outside professionals. For example if a child had speech problems then they would seek advice from a speech an language theraphist. Or if a child had mental health issues then they would seek advice from appriopate medical people.
"3) LSAs only work with SN and SEN kids, to my knowledge, at our school. Do LSAs work with G+T kids at some schools, though, is that what you're saying?"
It is very much up to the school and teacher. An exceptionally gifted and talented child would have an IEP (individual education plan) and targets. The school and the teacher can choose which children get one to one time with an LSA. It is very rare to a have child who is so academically so ahead to justify it though.
Yes, LSAs do work with G+T kids in a lot schools. Infact most schools LSAs work with small groups of children of all different ablities. A teacher will want to work with all the class at different times of day. Or an LSA will hear a child read on a one to one basis.