The SSN AUKUS submarine is a planned class of submarine that is currently under development by the UK, Australia and the US. It is intended to be a successor to the Astute class and Collins class submarines of the Royal Navy and Royal Australian Navy, and will fill the attack/hunter killer roles, equipped with torpedoes, and VLS cells for cruise and anti-ship missiles. The AUKUS class will be based on the Dreadnaught class currently being built by the UK. It's slightly unclear, but the Australian boats are likely to use a American combat system whereas the British ones will likely use their own. The British boats should enter service towards the end of the 2030s.

https://www.naval-technology.com/projects/ssn-aukus-class-nuclear-attack-submarines-uk/

The Royal Navy will have seven Astute class submarines (it currently has five in service and one remaining Trafalgar class submarine). If it plans to commission more than seven SSNs AUKUS, this would represent a naval expansion.

Resolves on a concrete announcement from the Secretary of State for Defence. Please note such plans can and do change - this market is concerned with intention. If the SSN AUKUS is cancelled and a replacement nuclear submarine programme is announced in its place, this question will apply to that instead.