You have nationality of UK and not of Pakistan? then you can not join Pakistan Army. Anyway if you are from Pakistan then following are steps to reach to ISSB.
1. Registeration through
Join Pakistan Army
2. Initial Tests
a) Written Test ( IQ and Academic ) online
b) Medical Test
c) Physical Test
d) Interview
after passing through these tests you may reach ISSB which is much more comprehensive.
Now it comes about if you should to leave your engineering or not. You don't need to leave your engineering. Just stay in engineering and apply in Pakistan Army commission. If you get passed then you may leave your engineering otherwise keep it going. If you get selected then there is no need to continue engineering if you really want to be an army guy.