Hi
Yes - Apple advises that you can buy AppleCare+ if you are within 60 days of the original purchase date of your Apple Watch, even if you did not buy it directly from Apple.
If AppleCare+ is available to buy online for your Apple Watch, without needing to visit an Apple Store, then it may be offered as an option when checking your service coverage here:
- Check Your Service and Support Coverage - Apple Support
- How to find the serial number or IMEI for your Apple Watch - Apple Support
Otherwise, to learn about more AppleCare+ and to obtain instructions for buying it in the UK, in a store, online or by telephone see the following page, including at the very bottom: