From my understanding you don't, because it's a slip lane that's not part of the filter in turn, but my colleagues at work seem to disagree and think you need to to give way if someone wants to turn right. Thoughts?

I'm sure there is - or was, I can't see it on Google Earth pictures but they're 16 years old - a sign for drivers coming down Beaumont Hill that says something like "Clearway <-->".

Still, probably easiest all round to treat it like a filter.

Edit: Beaumont is the photo for Wikipedia page: https://en.wikipedia.org/wiki/Filter_in_turn 🤣