Hello Qasim. Please go through the following thread.
How to Join ISI | Main Thread| | Pakistan Defence
ISI doesn't recruit as you would imagine. Your best bet is to join the Pakistan Armed Forces because that is where they recruit most of their staff from.
They do hire contractors and civilian experts for other roles. If you decide to join Pakistan Armed Forces, you would have to renounce your British Citizenship upon winning a commission. As a son of Pakistani citizens, you are able to claim a Pakistani passport. Training is tough, pay is reasonable and it will change the way you think and live.
If you wish to apply as a civilian, please refer to the above thread.