The postage for sending a watch to the US, from the UK via Royal Mail, using standard International Airmail,costs £3.30 -- see these two examples, many details below.
See more details of these packages, below. I live in Chicago, USA. In October, 2018, I received two packages from two different individuals in different parts of the UK. Each package contained a wristwatch. In both cases, the seller sent me the watches in simple bubble envelopes, with some internal wrapping of the watch inside. In each case, the shipping charge was exactly £3.30. The total charge from each seller was about £5-6. This was for International Airmail Standard. The items were received perfectly. They took less than a week to arrive. Below are photos of the full front of the packages, with personalized information redacted. (More, below)
You can see that both of these have been sent to Chicago. The postal code / ZIP code is on the packet. For the one packet with the machine stamp, you can see the date, 15 October, 2018. On both, you can see the price of the postage is £3.40.

I created this page to show how a simple item like a watch can be sent from the UK to the USA without great expense. £3.30 is the postage charge, at least at the time I am writing this in October 2018, for "International Standard," which is airmail, without any special services like "signed for," or "recorded delivery," or "express service" or anything special. It does appear to come with £20 insurance maximum. As mentioned, the items arrived quickly, under one week. And this was with NO special services, expedited, etc. These watches I purchased were inexpensive items. Certainly, a buyer of a more expensive item may request to have more special services like paying more for higher insurance, or paying for express mail, or paying for recorded delivery / "signed for." And in those circumstances, like if someone were buying a Rolex or Omega watch, I would expect a more specialized delivery service, and of course a higher charge. But as shown by these examples, and more (these are just two I highlight here) a small item like an old cheap or modestly priced wristwatch can be sent from the US to the UK without great expense. In each case the postal charge was exactly £3.30, and each seller charged total about £5-6, to include presume cost of envelope, and other minor et ceteras.

Thank you.
Alan

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