Don't despair - here's the same strap, same demo box for £21 - just bought one myself (ignore the first picture!)
Although this strap is designed for the 42mm Series 1-3 Apple Watches, it will also fit the newer 44mm / 45mm Series 4 & later watches as well. The item is still genuine Apple though. The box may be very slightly tatty (minor dings on the corners etc from storage), but this will not affect the...
www.ebay.co.uk
(actually, a bit annoyed I didn't think to look for this before, as I just leapt on a new listing for a 38mm one for £25 - looks like that'll be going back on!)