Apple watch trade in

You can trade in your watch for credit toward whatever your next purchase may be. For more info (to see how much your watch is worth) see:

Apple Trade In - Apple

Altear wrote:

willing to pay a max of £20 Xtra

That cannot possibly work.

I’m in the U.S., but I can tell you that here, the most an Apple Watch Series 1 would fetch is 10% of a basic Apple Watch Series 3’s price.