Why Britain could accept a stronger Soviet Baltic Fleet

Would Britain really accept Soviet naval parity with Japan and greater Soviet strength in the Baltic?

Yes, because Britain had limited direct Baltic interests and preferred treaty limits on Soviet construction to an uncontrolled arms race.

Britain had little direct interest in the Baltic at this point. British negotiators could leave nearby powers to answer the immediate regional balance while using the treaty to cap Soviet construction. Excluding Moscow would risk an unconstrained program and the wider arms race the conference was meant to prevent. Accepting a larger Soviet allowance could therefore be preferable to leaving Stalin outside the agreement.