And herein lies a potential problem.
The UK is not in the EU, both the EU and the UK will find it very difficult to diffentiate between non domestic sources (local origin rules etc) so both will need simple regulations which are most likley to be based on a binary "in or not in" rule.
If the "low value, duty = free" law is changed and the UK mirrors the EU it will cost consumers in both areas both money and inconvenience. It's not the duty per se, its the administration costs to calculate and collect that duty. Plus of course the shipper's profit margin. Your low value parcel of machine spares from (say) Germany to the UK will come more slowly and cost £20 in fees even if the value is only £10 and the duty is only £1.50.
Protecting local producers may be a legitimate objective, but it's coming 50 years too late. Many are no more, and we depend on global trade to get what we want/need. Reguation to stop or limit Shein and Temu will affect us all.
The US has thrown a pebble into the pool, the ripples will reach us all.