I used to have this problem at major stations like London Bridge. Full 4G signal bars, but no actual throughput. Seems to have gotten better lately, not sure if that's the effect of 5G or reduced passengers due to WFH.
I think part of the reason is that 3 only use a handful of bands where EE has a bunch more to spread the load.
Was the same when I needed to tether my phone, always had no signal but couldn't load anything. Had to hotspot off colleagues phones. This was all over central London (Shoreditch, London Bridge, Holborn)
Moved to EE and now don't have any issues.