It depends on what you buy.
I use 2 phones. I had a OnePlus 8 and a Poco F3. Managed to get good deals and went from the 8 to the 8 Pro and from the F3 to the F4. I got better cameras, but from a network point of view it wasn't an upgrade.
Recently I bought a Galaxy S23 Ultra. You can see the
screenshots here where it was getting 100Mbps higher speeds than the 8 Pro. It aggregates more bands and gets higher speeds every time.
Your testing phone isn't that great, so if you upgraded to something better, I think you might see improvements. Something with a Qualcomm SoC and their X65 (Snapdragon 8
+ Gen 1) or X70 modems (Snapdragon 8 Gen 2) since they've improved a lot in the past 2 generations.
The problem is that phones with these modems cost more than £99... looking at Xiaomi's website, the cheapest with a X65 is probably the Poco F5 Pro and it costs more than £400. At that price point you could get a second hand Galaxy S23 with a better SoC and the X70 modem my S23 Ultra has, plus a USB 3.1 port in case you want to use ethernet. I believe both are rootable (the Samsung probably a bit harder to root) so you could even have Network Signal Guru installed.
Only you know if you need this or can justify the investment. If all you need is to check the available bands/map areas and see if speeds are acceptable, then maybe you don't need a new phone.
Essentially, you need to actually upgrade to see some differences. Going from that phone to another like it will be more of a sideways upgrade on the network side and not worth doing it.