From the gov.uk website:
It’s illegal to hold and use a phone, sat nav, tablet, or any device that can send or receive data, while driving or riding a motorcycle.
A smartwatch can send or receive data. It is on the driver's wrist. Is it legal?
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I'm having trouble finding one page with the most up to date version of "Road Vehicles Regulations" with all amendments included, but the important amendment for this question is from 2003. Revisions to sections 4 and 6 were made in 2022, I've included the updated language in brackets.
“Mobile telephones 110.—(1) No person shall drive a motor vehicle on a road if he is using—
(a)a hand-held mobile telephone; or (b)a hand-held device of a kind specified in paragraph (4).
(4) A device referred to in paragraphs (1)(b), (2)(b) and (3)(b) is a device, other than a two-way radio, which [is capable of transmitting and receiving data, whether or not those capabilities are enabled.]
(6) For the purposes of this regulation— (a)a mobile telephone or other device is to be treated as hand-held if it is, or must be, held at some point [while being used] ;
A smart watch definitely falls into the category of interactive devices in paragraph 4, so the question is whether you "hold" a smart watch while using it. The plain English answer would of course be no, you normally wear a watch while using it rather than holding it. That doesn't necessarily mean that a court hasn't interpreted "hold" under a definition that includes a wearing smart watch though, don't take this as legal advice.