I once posted, and then relisted recently, an 'urn' shaped piece in what I thought might have been S. & W. 'Jade' It had handles in this watery white colour - and like yours also had a flat base. There was a bit of a deafening silence in terms of replies, so in the end I became no wiser really. However, in recent weeks I did in fact find what Nigel Benson confirmed as a piece of genuine S.& W. Alabaster, albeit the bottom half only - and this has a rather large ground/polished pontil mark, as shown in the attached pic. So maybe flat bottomed examples are not from the U.K. Frederick Carder apparently did something similar to 'Rose', and so did Richardsons, I believe, oftern copying S.& W. patterns.