Glen — your crown version solved the mystery. I have the exact matching design on a commemorative casserole lid in a boxed set, complete with most of the original label on the box. Priced at an astonishing 27/6, this set is a lidded two pint round casserole in a pierced aluminium stand with two ivory bakelite handles — The NEVVACOLD Royal Coronation Model. The Phoenix brand casserole was by the British Heat-Resisting Glass Company Ltd., originally of Lodge Road, Birmingham when formed by ex-Chance employees after the abandonment of Orlak production, but by the time of the Coronation of Loxdale Road, Bilston, according to BGbtW. Lid marked "RGG 500L", Base "RG 500B" plus the Phoenix TM, aluminium stand unmarked.
So, Ivo and Glen, your coasters are Phoenix heat-resistant glass, specifically intended for use with hot drinks.
... and I can't help wondering if there was a connection with Walsh, also in Lodge Road.
Bernard C.
Later:
A Google search found this Phoenix webpage, further confirmation of the attribution. Note the dangers of over-enthusiastic restoration of a label using software tools. My label clearly has the original "27/6", not the misleading "7/6" shown here. A useful reminder that an unmodified photograph of the original label would have been a wise addition to this page.