Is there any International Hub in UK?

What they were trying to tell you is that there is no way to get directly to Plymouth from outside the UK. In order to get there from Hong Kong you will therefore have to fly to another UK airport and travel to Plymout from there. From Hong Kong, you can catch a flight into Heathrow, Gatwick, Stanstead or Manchester.
For more information, check out the website for Hong Kong Airport at http://www.hongkongairport.com/eng/index.html

In order to send your letter, the courier company send it from hong Kong to a central point (a hub) and then on to the final airport (somewhere in England), where they will then take it by road to Plymouth.

This means your letter is en route to Plymouth but is not there yet.

BIRMINGHAM

London or Gatwick