one thing I've noticed since 5g has been introduced in the UK is that 4g has gotten significantly slower and less prevalent. has anyone else noticed this?
Not at all - in fact what I have noticed is that 4G seems to be far more reliable than 5G (on a Galaxy Fold 3 here) even in strong signal areas.
In the US, carriers are allowed to brand an enhanced LTE service as "5Ge." That may account for there being fewer times when your phone shows "LTE" or "4G" as the connection type.
That's not unexpected after a while, as the telco switches prioritisation and bandwidth from 4G cells to 5G cells, as happened with 3G to 4G. But it's too early for it to be happening at a noticeable level and might mean your provider has underinvested in capacity.
Just a guess though.
Many phones (certainly iPhone 12 behaves this way) will show 4G LTE link as being "5G" when they are connected to eNodeB that is capable of 5G in NSA mode regardles of whether the link actually uses any 5G NR bearer channels or whether it is actually physically possible (interference, path loss, gNodeB antenna configuration...) to use NR channels.
Thus you can pretty well observe a lot of what really is 4G as 5G.