"No one has mentioned anything about what we are buying other than that it will be nuclear powered."
Given that it is going to be British or American there really isn't much choice - waiting for a new design would push back in service dates by over a decade. Taking a current design and modifying would be a very bad idea (see numerous posts above).
Given the small number of "Astutes", the slow roll -out rate and the need to start building new SSBNs in the UK, it looks very likely to be a "Virginia" - possibly second hand to start with. Still a lot of punch for your dollar