EPW013595 ENGLAND (1925). The Harbour, Pavilion Theatre and sea front, Weymouth, 1925

Two of them, the Isle of Jersey and the Isle of Guernsey, were built for the Southern Railway in 1929 by Wm. Denny & Bros Ltd of Dumbarton, who were noted constructors of ferries. The similar Isle of Sark was launched from that firm's slipways in November 1931. They finished service circa 1960.

[Source: Richard Danielson, Railway Ships and Packet Ports, Twelveheads Press, 2007, 49-50]

I think this is the GWR Channel Islands steamer "St Helier", which was brand new in 1925. The Isles class of steamer were as you said built in the 1929 -31 period but for the Southern Railway services from Southampton. Weymouth was in those days a GWR port.

See comment below