Which application SET(O) or (F)

[vinodkbg]

Hi
I (main applicant) and my wife (dependant) switched from Tier 2 (General) 5 year route to ILR recently (got ILR on 29th March 2019). We used the premium service for our application.
To save a bit of money we decided to apply for our 2 kids's ILR separately via post.
Now I am preparing their application. Both my kids currently have Tier 2 general dependant visa (expiring on 19th of July 2019) and lived here for 6 years with that visa (obviously with one extension).
Would you please let me know which application form I should use SET(O) or SET(F)?

Is it OK if I apply through postal route or do you recommend premium service?

Thanks in advance.
Vinod

[zimba]

SET(O)
There is no postal route anymore. Apply online and via UKVCAS