What size grand are you looking for? It's not surprising that the market has more Yamaha (and maybe Kawai) grands as they will likely have the largest number of new piano sales. I was offered a new C3X last February for 22k so not too surprised that these as well (as the GX3) might hit the used market in a relatively new condition and would be well within your budget.
If you're looking for a German/Yamaha SX/Shigeru level piano, then your budget is likely just a little short but there are pianos that do come up close to or sometimes without your budgethttps://thepianogallery.co.uk/videoshop/schimmel-k189/
I was seriously considering this Schimmel before I found my grand. There was also another Schimmel that was from a private seller in Nottingham for a C189 (2003 if I recall) that was under 20k that has now been sold.
Vale Pianos also have a couple of grands that are just over your budget. I know prices aren't on their website but I looked at these at some point toohttps://valepianos.co.uk/pianos/pre-owned-pianos/grand-pianos/rud-ibach/https://valepianos.co.uk/pianos/pre-owned-pianos/grand-pianos/schimmel-189-konzert/
Of these I preferred the Ibach when I played both but worth calling them and trying these
If you can stretch to mid to high £20k+ then this probably is as good as it gets (not biased or anything here)https://shacklefordpianos.com/colle...weg-g225-grand-piano-in-black-high-gloss
I called up more for personal curiosity really. The piano serial suggests it was made in 1990 so pushing your 30 year age limit a little.
When I was shopping 2020 to 2021 I saw a couple of pianos from Chamberlain's that I would have loved to play (but because of the lockdown didn't get the chance to and were sold to buyers during the lockdown). One was an SK2 that was 15-20 years old that was priced just over 20k that had some wriggle room (called up and told could get this under 20k) and a Yamaha S6 (20 years old) that was 24kish.
I would say the under 30 year old market for a something like a recent German or equivalent Yamaha/Shigeru probably starts at 20k having spent a lot of time looking at it till early last year when I bought my grand.