Well, there's no doubting the date letter, 'U' used 1775-1776.
Grimwade suggests that this mark, if with a star, may be that of John Scofield, but if with a pellet, it may be that of James Stamp. Neither mark is traceable in the registers.
Grimwade states that Scofield's first mark, that of his partnership with Robert Jones (R.I over I.S) was registered on the 10th February 1776, this would appear to rule out Scofield as the maker, as his second mark, alone, was not registered until 1778, but little is known of Scofield's earlier period, if any, and it is quite possible that the star mark was registered prior to his partnership and was contained within the now lost Largeworker's Book (1758-1773). Whether or not Scofield did appear in that book would only be speculation, but one thing that makes it unlikely is the fact that Scofield was likely to have been the junior partner to the perhaps elderly Jones (Jones is thought to have died before 1783) and with Scofield's undoubted skills, it would seem unlikely that he would depart from a solo career to joined such a partnership.
James Stamp, however, was a prolific silversmith and working alone at the time your mug was assayed, he entered several marks and on the balance of probability, perhaps a more likely candidate for the maker of your mug, but it would be impossible, I believe, to say so with any degree of certainty.
Hope others will add their opinions.