Yes, our sovereign parliament can ignore any international treaty it chooses. So if
Parliament were say, to withdraw from the GFA, then as a matter of U.K. law the GFA would no longer have legal force. However in the real world, unilateral withdrawal from international agreements has consequences. So if the EU fishing fleet started ignoring our territorial waters, we’d sink them. Or, more realistically, if we put in jeopardy the GFA, the US would be unlikely to agree a trade deal with us.
The reason is that Parliament is the supreme arbiter of what is lawful within the U.K. So if it says blue eyed babies are no longer lawful, that’s the law.