Just a few observations on this thread
Just to throw this info into the mix ...
My postie confirmed it's a random pick from the mail which enters the country as to which items get checked and which don't ... anything marked over $30 will get put to one side and be charged ... like mine was.
Since 9/11 almost all packages are checked
Anyway, he then told me that this would be a thing of the past shortly, when Royal Mail introduce an automated sorting machine which is proven to be capable of sorting
ALL the mail prior to delivery ... letters, packets, parcels, handwritten or printed addresses and labels... it doesn't matter what it scans as this machine will read the lot.

Think about it - at the moment it's a hit-and-miss affair whether you get charged on your items coming into the UK or not and £18 in value isn't a very high limit is it - you won't have that to fall back on once this machinery is up and running as
everything will be scrutinised whether you like it or not!

The implications for the postal staff are wide-ranging and not good - there will be a massive number of people being made redundant and sorting offices WILL close ....
I've not heard of anything about it in any of the Press here in the UK and believe me, there will be such a stink once it gets out about the redundancies.

The machinery itself is hugely expensive and I believe the Dutch mail service already uses this automated system to good effect. You won't need an army of people hand-sorting the mail prior to delivery, just a couple of folks to load the mail in one end and and bag it up at the other ....
We have been waiting for machines that live up to there promises in Royal Mail for years. It may happen, but these machines usually only perform with perfect mail and I doubt that the machines will be reading customs forms anyway.
As for declaring the correct amount on customs forms, if you're surcharged at least its a more secure form of delivery than the signed for services. Registered mail coming into the UK is now sorted with the normal mail (although it should still be signed for), instead of being transfered from office to office by a more secure system.
And before anyone complains about this to the Post Office I can assure you the staff have already done so, but they have been told that this is by order of the Postal Regulator.