Herbert Henry Asquith
The longest-serving PM of the 20th century until Thatcher, Asquith's time as PM was interesting to say the least. In his lifetime he was always referred to as "H.H. Asquith".
Succeeding the late Henry Campbell-Bannerman, Asquith's government created the UK's National Insurance system, took the UK into World War I and faced two rather large domestic crises.
The first was over Home Rule in Ireland, an issue which the Catholics favoured and the Protestants strongly opposed. Home Rule was halted in its tracks by the war.
The second was the 1909-1910 Budget Crisis. A rather redistributive budget was vetoed by the House of Lords, sparking a constitutional crisis. Following two elections in the space of a year (which only succeeded in making the Liberals a minority government reliant on Labour and Irish support) a deal was reached in which the House of Lords lost its powers over the Budget and could only delay legislation by two years in a piece of legislation called the 1911 Parliament Act. This was later reduced to one in 1949 and the later Salisbury convention (the Lords doesn't block manifesto commitments) followed.
Asquith was eventually forced out in 1916 and replaced by David Lloyd George.
He's the great-grandfather of Helena Bonham Carter.
Known for being perpetually drunk, especially whilst giving speeches, which apparently was no great impediment to his political career. Inevitably nicknamed "Squiffy" in an Incredibly Lame Pun.