Though it appears counterintuitive and inefficient, AUKUS makes perfect sense for the UK when seen in the context of the country's post Brexit vision and priorities. Though much of the leave campaign was driven by xenophobia and concerns around immigration from Eastern Europe, the stated aim of Brexit was to unleash a buccaneering, 'global Britain' free from suzerainty to Brussels. An alliance with Anglophone nations sharing the same head of state, legal and cultural traditions- the so called 'CANZUK'- has long been regarded by Brexiters as mitigation for the loss of UK's global influence from exiting EU. Not only would the alliance create the 3rd largest economic block globally with nuclear weapons and a seat at UNSC, it would also provide the basis for projecting British, or rather 'anglo saxon', soft power across the world. This block will also work in close partnership with 'like minded democracies' (read US, Japan, possibly India) to resist the rise of China and Russia. The EU is not considered a reliable partner in this regard given its mercantilist impulses and internal dissentions particularly from Eastern European states that make presenting a united front to China and Russia difficult. The NATO is also no longer seen as fit for purpose given Turkey's undermining of the organisation in recent years. Since future conflicts are unlikely to see deployment of physical armies/equipment and with the advent of hypersonic/low earth orbit ballistic missiles and submarines with unlimited range, one could also argue that geography is becoming increasingly immaterial for such alliances.
From an electoral perspective for the Tories, whilst some of EU's recent actions have helped keep the Brexit pot simmering, the party's base is getting restive and will demand to see tangible benefits from the newly gained 'sovereignty' at the next elections. Further strengthening of ties with Oz, with which the UK shares deep fraternal ties, is certain to be electorally popular in middle England. There is also a new trade deal being negotiated between UK/OZ which will, among other things, reinstate limited freedom of movement between the two countries that was lost under the Maastricht treaty- so AUKUS is being seen as part of a wider piece in this regard.