From today's Guardian:
By the way, there is an awful lot of comment on social media by angry politicians that the Chagos Islands agreement should have gone through parliament. All international treaties have to be ratified by parliament, so there will clearly have to be a debate about it in due course, and it is somewhat disingenuous by those in certain quarters who appear to be trying to suggest that parliament has been bypassed.
"Under the provisions of the Constitutional Reform and Governance Act 2010, all treaties (defined as written agreements between states or between states and international organisations which are binding under international law) must be laid in each house by a minister, together with an explanatory memorandum. The Foreign and Commonwealth Office also sends a copy of the treaty and the memorandum to the foreign affairs committee, and to relevant departmental select committees."
Ratification of treaties - Erskine May - UK Parliament