Okay this is the thing. If my UK visa expires in the 3rd week of Oct, but im coming back to Lanka in Aug till like 1st week of Oct, should I renew the VISA from Lanka or UK?
Which do you think is better ? Applying from here or the UK?
Apply new visa before you come back to SL. If it's Student Visa you'll have to do IELTS again (Min 5.5)

better to renew & visit SL
