I think basic education (perhaps up till Class 8) should be in Urdu because :
(i) its the closest one can get to an indigenous language while still maintaining some semblance of uniformity in curriculum across different Provinces & even different Districts within the same Province.
(ii) every Pakistani is, in varying degrees, fluent in Urdu.
(iii) there is a technical & standardized version of Urdu whereas our regional languages aren't even Standardized in the same Province let alone having a technical version of them; for e.g the Punjabi of Lahore & Jhang are considerably different than each other.
(iv) our Provinces aren't homogeneous when it comes to languages - Far from it; e.g in Punjab the Southern parts speak Saraiki, Baluchi & Punjabj, the Central parts speak Punjabi & Urdu, the Northern Parts speak Gojri, Potohari & Punjabi whereas the Western Part speaks Pashto, Saraiki, Punjabi & Hindko - Which language should be the medium of Instruction ?
Thats why I think that no other language but Urdu makes sense to be the Medium of Instruction & in the mean time English is taught as a Second-Language properly so that proper Comprehension Skills are built in the Students & they are comfortable with the English Language once it becomes the Medium of Instruction after Class 8. This would be a good choice, in my opinion, because:
(i) thrusting English on our children at a very young age when most of them aren't fluent in it at all as opposed to most being fluent in Urdu, in varying degrees, would affect their ability to develop those fundamental concepts needed that further studies are built on...just because they haven't a grasp of English & you can't really teach
the English Language & teach
in the English Language simultaneously & hope that the student picks up.
(ii) English is needed for higher studies & teaching English as a Second Language, in a holistic manner, for nearly 8 years before you're asked to use it as a Medium of Instruction, would ensure that the student's ability to understand English thoroughly, be comfortable with its usage & perhaps even learn to think in English would be established.
The local languages are a part of our culture & they must be cherished - I think everyone ought to be mandated to learn the basics of at least 2 local languages till Class 8 & then choose any one of them for further studies !
Just my two cents !