@Kiwigirl - despite what you were told upthread, this has
nothing to do with the EU (its forerunner, the EEC didn't come into existence until 1957!).
The core principles of how refugees / asylum seekers should be treated were determined by the UNHCR (that's the
United Nations High Commissioner for Refugees) Convention that was created in 1951 and the UNHCR Protocol of 1967 which reaffirmed and and widened the scope of the Convention). The UK is a signatory to both and has incorporated it into its own laws. Incidentally New Zealand (which I assume is your home country) is also a signatory to both the UNHCR Convention and Protocol.
There is also a wide range of international law that prevents any country from simply pushing boats back into the sea. The people that arrive on the boats could be returned to the country from which they embarked or their home country but that would require the UK have an agreement with those countries and on the whole they don't exist.