You can't have the same number but some watches will clone to your phone so in effect you are using the same number but it depends in the type of watch.
Also, a sim isn't 100% necessary in most watches so a PAYG sum would be your cheapest option. Monthly bundles are from £10 a month or just top up as required on the classic PAYG tariff.
https://www.o2.co.uk/shop/sim-cards/pay-as-you-go/
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