I believe that you'd find that you're dropping to B20 when the signal strength increases but the throughput reduces. B20 for Three has only a third of the theoretical capacity as B3 (your screenshot from the B353 above shows CellID 13018625 which translates to Band 3).
However as B20 travels further and penetrates buildings better it will cover a larger population and so that third of the theoretical capacity gets shared more.
If you note the CellID that your B353 shows when the signal is high and the speeds are low you'd know for sure.
Why this happens could be a number of factors, including but not limited to; environmental factors (new trees' leaves/buildings blocking signal paths), network reconfiguration of thresholds that decide which frequency layer your device should be using or increased interference driving down your metrics and pushing them into the lower ranges of the network thresholds.
To try mitigate it you could move your router to other places in your house where there you can find better signal levels on B3 - probably preferable to ensure line-of-sight to the mast (or with minimal obstacles, e.g. walls) - the site seems to be at Clay Hill, Great Henny, CO10 7NQ. Or you could maybe use band locking tools (though I'm not sure if any are available for the B353, others may chime in here) to exclude B20 as an available band, and therefore effectively always use other bands that are available (the site has B1/3/20)