Some sellers are of course not willing to declare the goods as you suggested or not send uninsured or not send to the UK at all.

I receive most goods as 'gifts' and then 'self declare' them. I can do this as I have an EU import license. If an item is declared in full or stopped by customs this causes two inconveniences.

1. Minor fact of having to pay COD for the duty and then reclaim it at the end of the quarter

2. Major delay in customs of up to 3 weeks during which on-line tracking records disappear.

IMO a lower value draws less attention from light fingered workers

IMO large boxes only ever go missing if incorrectly addressed etc.

Small envelopes declared at high values are the most likely to go astray.