Does domestic or international law take precedence in the UK?
I ask, because the treaty agreed with the EU for the Brexit extension appears to override the European Union (Withdrawal) Act 2018.
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Treaties have to have enabling legislation in the United Kingdom. They do not have direct force of law by themselves. There is no written constitution, and the purported hierarchy of laws given in another answer is incorrect. In fact, U.K. courts do not even usually consult the texts of the treaties to determine what the law is; it is firstly and foremostly taken to be whatever the enabling legislation says, with treaties and other documents only consulted if the enabling legislation is ambiguous or open-ended.
The European Communities Act 1972 was one such piece of enabling legislation. You can see the specifics of enabling the treaties in its § 2. The European Union (Withdrawal) Act 2018 repeals it.
The latter Act specifies that this happens on what it terms "exit day". Like many Acts, it has an "interpretation" section for its jargon terminology, in this case § 20. That section defines "exit day" as 2019-03-29. But it also provides a mechanism for "exit day" to be altered by Statutory Instrument, if the treaty processes end up with a different day actually being exit day. The normal procedure for SIs is for a Minister of the Crown to lay them before Parliament, and Parliament approves or rejects them. If approved, they become law.
And that is what has happened. The SI amending exit day went before Parliament yesterday evening at 21:00 GMT. It was approved, by a majority of 441 votes to 105 in the House of Commons at 21:19 GMT. (At the time of originally writing this answer, the WWW site publishing the SI had not been updated to reflect its approved status. Technically, both houses of Parliament must approve it, and I was still waiting on the House of Lords. I noted that was extremely unlikely to be blocked in the Lords, and most people are regarding it as de facto approved.) It was approved in the House of Lords the same evening, and is now listed as made at 12:40 GMT today, 2019-03-28.
The generally accepted hierarchy of Public Law is:
Constitution or other Founding Document
Treaties/International Obligations (Customs), i.e. International public law
National Public Law, in whichever order applies
See for instance:
The UK is a little different, since it has no Constitution or Founding Document. JdeBP's excellent answer has the procedural details on how this works in the UK.
In the specific case of the EU's body of treaties and laws, the UK position was clarified during the legal saga of Factortame Ltd versus the U.K. transport secretary, which ran from 1989 to 2000 over fishing rights. It saw the House of Lords confirm that EU law was supreme over British law in areas where there are competencies in the EU treaty.
Surinder Singh is also worth a note in passing. It basically ruled that EU free movement laws override UK Immigration laws in Singh's specific case.