PLAB 1 exam is conducted in
Pakistan and you may want to sit for PLAB1 there itself and then come to UK for the PLAB 2 exam.
Anyhow, there's no PLAB visa anymore, so if you intend to travel to UK to sit for either of the PLAB exams, you'll be applying under the visit category only. Please note visit visa cannot be extended from within UK and on a visit visa you cannot change to another immigration category from within UK.
The employers (NHS / Deaneries / Trusts / etc.) have nothing to do with the type of visa you came to UK on to sit for the PLAB exams. What they will be concerned about is your immigration status in UK when you apply for jobs (after you pass your PLAB and get registered with GMC).
If you intend to come to UK under the PBS scheme (e.g. - Tier 1), please be advised that you will not entitled to employment in training posts (e.g.- Foundation Programme).
regards