- Does he need to take LIUK test as he is over 18 now?
Yes.
- Though he has been studying in the UK for over 11 years, I saw some posts on the forum that he needs to take English Language test. Is this correct please? He already has GCSEs and will have A level results and certificate. Are these sufficient enough to prove his English Language ability? Please advise
He needs a ukvi approved B1 English test.
- Which form should I use to register as British citizen?
Form AN, and same form you used.
2) My younger child is 14 now and has been on ILR for 1 year now and has lived continuously in the UK for the last 11 years. He too has studied in the UK for 11 years
- Do he need to take LIUK as well?
- Do he need to take English Language Test?
- Is it possible to register him directly as British Citizen and not go for naturalisation? If yes, then which form should I use please? If not, then which form should I use?
Children under 18 do NOT need LIUK or English tests!!
Children under 18 are REGISTERED as british. Use form MN1. Only adults are naturalised!!