fwiw, I posted some iperf results running latest snaphot on HH5a
https://forum.openwrt.org/t/very-slow-connection-as-wifi-client/119415/20
Have you tried using the TalkTalk router for a few days to rule out issues with the incoming GFast or Talktalk service?
The HH5a at end of the day is too slow when running OpenWrt stable for 100+ mbps services. You need a faster device.
Nevertheless, those awfully slow speeds and 3 figure ping times don't add up. The HH5a is slow, but not that terrible!
Any recommendations for a more suitable router?
Plenty of low cost Mediatek MT7621 (wifi issues aside) routers available. There were even some open box Xiaomi R4A Gigabit for less than £19 last week on ebay from a UK seller (£32 new from Xiaomi UK). I think Archer C6 uses same chipset. The R4AG achieved 320 mbps when I tested client mode wifi the other day in openwrt using iperf3 between two laptops.