No, all restrictions that are required by law in Japan, will still be active in any other country, since the restrictions are part of the country specific watchOS, which can not be changed.
Also, if you buy an Apple Watch abroad, these devices are supposed to be used in the country of purchase, due to the different built in hardware, that has to support local carriers, especially if it comes to LTE.
So make sure that your carrier in the UK supports this model and all its features.
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copied from: Apple Watch - Carriers - Apple
Remember, that the warranty for an Apple Watch can be restricted by Apple to the country of purchase.
In case of a warranty claim, you will have to bring the watch back to the country of purchase, Apple Support does not ship internationally.
Legal - Apple Watch Warranty - Apple