Second-calendar year Baltic Gulls have a different primary moult strategy to the same age Lesser Black-backed Gulls (graellsii and intermedius). Most Baltic Gulls moult during their time in their winter quarters and so arrive back in Europe with a fresh set of second-generation primaries. In contrast Lesser Black-backed Gulls moult their primaries in September. So basically second-calendar Baltic Gull is identifiable on primary moult timing. There are no known photographs of either graellsii or intermedius with a full set of second-generation primaries as early as July. Therefore I feel confident to call this gull a Baltic Gull. This bird was accepted by the BBRC in 2018.
Thanks for looking.