The British Library has a large number of Yiddish books and periodicals in its holdings. Some information on its Yiddish holdings can be found on the following pages:
"Aside monographs, the collection comprises over 650 Hebrew and some 250 Yiddish periodical titles, with approximately a third of the total being current subscriptions. The collection of Hebrew periodicals is of particular significance not only on account of its impressive geographical spread, but chiefly because many of its titles represent milestones in the history of the Hebrew press. Noteworthy among the Hebrew journals are : Kohelet Musar [Ethical adviser] the first Hebrew literary periodical issued in Berlin about 1750, ha-Magid [The Preacher], the first modern Hebrew newspaper issued in Lyck and in other places between 1856-1903, ha-Levanon [The Lebanon], the first newspaper issued in Jerusalem and afterwards in Europe between 1863-1886."
"The Yiddish periodical collection is especially strong in American publications, many of which are held in microforms. There is also a fair number of 19th and early 20th century Yiddish serials from England and Eastern Europe in the collection. Di Tsukunft [The Future] and Forverts [Forward] issued in New York in 1892 and 1897 respectively, Loshn un Lebn [Language and Life] a London literary published 1946-1981 and edited by Abraham Stencl, and Der Idisher Ekspres [The Jewish Express], first published in Leeds in 1896, are just a few examples."
See furthermore also this overview: Jewish studies: sources in the British Library Slavonic and East European Collections.
The library's Integrated Catalogue can be consulted here.