Thing is, the British constitution doesn't have such specific provisions for Muslims.
The constitution of Pakistan is very specific and clear on the matter of the Qadyanis. So, before you open your gob, understand what your country laws are saying. Either obey or piss off from these lands and surrender your passport. Savy?
I personally do not have issue with his appointment, but since now it has become a political issue, the last thing PTI government want is the likes of Khadim Rizvi on their arse over this matter. Its simply doesn't worth the risk with this gentleman and move on to next best candidate. Unless, he comes forward, and declare his allegiance to the constitution of Pakistan, which he SHOULD anyway, being a citizen of Pakistan.
At the end of the day, its PTI own fault, putting themselves and this man in tight spot , should have done their back ground checks.