As you are an UK citizen you return home with items you bought in the UK, you have already paid the taxes on them. It might be hard to proof it, as you likely did not take the paperwork with you.
If you have online banking and a payment that shows this phone or watch being paid in the UK, you can keep that or a print out of it handy.
If you bought your phone and/or watch in the EU before Brexit, you may try to argue that you have paid the tax while it was still part of the same system.
Next time you leave the UK with new or rather new expensive items, either take the paperwork from buying it, or get proof you did own it before you leave the country. You do not need to pay repeatedly for the same items.
As a foreign tourist you would normally not need to pay for items you take with you on travels, as long as you plan to take them out again.
Again, having an expensive item and staying long in the country you go to, like as a UK citizen who has the right to stay in an EU country, bringing proof you owned it before this travel will help.
New phones and watches do go down in value quite fast and the border officials will not blink an eye if you pull out a phone which is a few years old.
If you buy a new phone and watch and/or other things that take you over the limit you need to go to the red channel, tell the officer what you bring and how much you paid, and if you can not show paperwork, they may assume a value, and pay tax over what you bring in. If you do not go through the red channel, they may take it off you and still fine you. If in doubt, declare and they will tell you what you need to pay, or nothing.
These rules are now new between the EU and the UK but have been normal for years for travels from either the EU or the UK to other countries and what I write here is standard, learned over the years, no links.