He might have a long walk to his nearest Halfords, but any varnish that claims to be resistant to temperature should be just fine as long as enough layers are applied to lose the groove and present a smooth surface.
Even something acrylic like Johnsons Kleer (they say that its floor polish, but we know better) should do the job. Of course, any heat from hot plates will go straight through to the vinyl, the varnish will probably stand more than that, but it will not protect from heat.