Going from an R7800 (owned and tested) you’d probably not see much real world difference, aside from benchmarking.. At least coming from my personal experience beta testing multiple high end routers including up to the latest RAXE500 and Orbi AXE line… and I’d personally save the money, I agree with sfx2000. If you’re unsure I suppose you could try whichever new model you are thinking of and see if it makes a real world difference in your environment…
Being a "modern flat" in the UK, what works best for you probably comes down to the internet entry point, what kind of internet you have, the construction of the flat, and how kind the developer was when it came to ducting!
If you have FTTP, and the flat construction is thick solid walls, with no available ducting then you might be looking at powerline to WiFi systems such as the Zen XP4 Hybrids (Home AV2) or Devolo Magic 2 (G.hN) WiFi systems. They'll get "decent" WiFi around the whole flat. The problem is that if your internet is provided using VDSL(2) these systems often have a major effect on that VDSL speed - it's a quirk to do with UK wiring and common construction of flats over here.
*Just one suggestion that I've not seen. Both HomeAv2 and G.hN work VERY well in single-ring main UK flats - provided they aren't killing ADSL/VDSL in the process!
Being a "modern flat" in the UK, what works best for you probably comes down to the internet entry point, what kind of internet you have, the construction of the flat, and how kind the developer was when it came to ducting!
Based on discussions with friends based in the UK - most of the telco's will drop their endpoints and gateways in a location that is convenient for the installer, and not necessarily for the residents...
Right now, just have 80mbps internet connection on the OpenReach network. I have phone lines in 3 different rooms, so the router, modem & LAN devices are in the most convenient place. Internet speed is pretty great for my purposes and can use 4k streaming services on the TVs without issue
FTTP has been physically installed but is not yet activated. Should be in the coming weeks, though i'm in no rush to upgrade. The entry point for FTTP is however more awkward (a little white box above the outside door of my flat)
I've found the R7800 great. It was the most recommended product on this site for a long time and good to hear it's still standing strong. Depending upon percentage gains, improved performance on my local network may be of value (lots of network storage etc)
Your router can do 550Mbps to common 2-stream AC client. The same is valid for currently most recommended AX86U. You won't get any improvements unless you replace your clients with AX capable. Most common AX client is also 2-stream, or you can get about 800Mbps. Some may recommend trying 160MHz wide channel, bit in an apartment building you have less chances of success and your clients must be 160MHz capable too. Not all AX clients are. From experience, R7800 may actually beat AX86U in range. As I said, R7800 is exceptionally good in range and is hard to beat.
