OK so I mentioned in my recent intro post that I take graded exams in keyboard (although it is with a different exam board) but study actual piano music outside of exam prep time. I enjoy playing the piano quite a lot and after a concert performance I gave last month I wondered if that is something I should perhaps enter myself for at ABRSM at a later date.

When I was 16 I was taking up Music Technology as an A-level and at the time my mum found a school for me to enrol on which teaches the keyboard (and vocal work, but I'm no singer!). I left when I started university but when I decided to return to this I told my teacher that I would like to reach Grade 5 in keyboard (I mean just start studying the material, not necessarily take the exam) before I graduate after which I could move away longer distance permanently for the sake of finding the best work opportunities. I am currently studying for Grade 3. She says I am perfectly capable of doing that, and that the piano piece I performed in the concert is at the top end of Grade 5. I think at a push I could prepare myself to play Grade 6 material.

How might grades in keyboard and piano compare? Would it perhaps be possible for me to walk in at, say, grade 3 or 4 for piano given my previous experience? Or would I need to start from the first one, still?

If you've been playing piano repertoire as well as keyboard you could take a piano exam at whatever level you are at. Remember to check the rest of the syllabus. Other exam boards do piano exams too so you could compare them for choice of pieces and for supporting tests.
It would help if you can get some practice time on a piano, either acoustic or digital as it would be an acoustic piano in the exam and I'm sure you're aware that there is some difference in touch an playing technique between piano and keyboard.
If your teacher is also a pianist then you could ask for help or you might want at least to find a reasonably experienced pianist friend to listen and advise.

Thank you for your reply. It seems like I could take on Grade 5 after another 18 months or so. I'll definitely consider it once I'm ready, and when university is out of the way.

In fact when I was on campus I was practicing a piece for my last concert on acoustic pianos quite a bit so know how they feel quite well. We do have an electric piano at home but the keys are horribly clunky - once I was playing and my mum was in the room below and she honestly thought somebody was hammering a nail into the wall. Although the school I go to teaches keyboard, they have an electric piano which they make available at concerts in case anyone wants to do a piano piece like I did. My teacher did say she would be getting a more up to date one and offered to give me a price for the old one. It feels lovely to play and I'd love one like that to practice piano repertoire at home with, so fingers crossed!