I suppose you have already checked Ancestry's All England, Alien Arrivals, 1826-1869? I don't have a subscription, but i can see a M. Loewenthal arriving in London.https://www.ancestry.co.uk/search/collections/1587/?name=_löwenthal&arrival=1859&arrival_x=2-0-0&name_x=iAnd this family tree?
Have you checked his business partner?"Henry Pallash and Morris Loewenthal, Aldermanbury. manufacturers of leather goods"Published: Wednesday 17 January 1866Newspaper: GlobeCounty: London, England