Yes, on March 31 1939 British Prime Minister Neville Chamberlain issued a statement guaranteeing Poland’s sovereignty, this was followed up by a preliminary agreement on mutual defence on April 6th and a mutual defence alliance on August 25, 1939. The discussions and text of the agreement and alliance are contained in the British War Bluebook. Note that the alliance committing Britain to aid Poland in the event of attack contained a secret clause specifically stating that this referred only to an attack by Germany and not by the Soviet Union; this secret clause is not contained in the Blue Book and is actually extremely difficult to track down.
France had a mutual defence alliance with Poland from 1921 onwards, but this had been considered by the Poles to have been abrogated by France’s signing of the Locarno Treaty with Germany in 1925 and was only renewed again in early 1939 following German demands and in conjunction with the British commitments.
Note that the Franco-Polish alliance had been very specifically designed to deal with the threat of the Soviet Union; part of France’s Cordon Sanitaire of the new Bolshevik regime in the early 1920s, as well as to deal with Germany. Despite this, the French government did not consider a declaration of war on the Soviet Union following their invasion of Poland in mid September ’39.