I believe you indicate when you apply but can make changes after you start, with the limitation that not all colleges offer all three subjects of PPL (which would also require/suggest you make this decision initially, as if you apply to PPL at one college but want to study the L specifically, and they pool you to a college which only offers PP, you're kind of stuck), and so some will only offer Psychology & Philosophy for example, or Psychology & Linguistics. PP was the precursor to PPL so I believe in general it's more common for a college to offer both Ps but not the L than any other combination, so you're probably less restricted than others. The university admissions website indicates MLAT is for Linguistics only, so if you apply to PP then I doubt you will.
That said, I imagine most students have no actual linguistics experience (in terms of that as a discrete academic subject) and not all students will even have languages experience (e.g. those applying only to the Linguistics courses) so I doubt it's structured in a way so as to depend on that. The admissions pages indiacte that a language is only "helpful", and not preferred nor required (and it's also listed as such alongside Mathematics and English Language). Overall I don't think you have much to worry about either way, but it looks unlikely you would need to take it.
If you are excessively concerned, you can always contact them directly to confirm whether any of the above is correct.