OK, so I wrote a long answer but Youtube crashed my display drivers so I lost it.
Short form: H2/O2 module is 3m by 17.6m. Equipment inside is in acoustic enclosures so takes up more space. We actually thought it was a tight fit. Not necessarily the most efficient arrangement, but this was just a starting point. Remember the aim of concept studies is not to provide the answer, it is to help determine the question that must be asked. PH diameter is 9.4m aft and 6.8m fwd where it is surrounded by the UUVs. Complement is 72. 7 arrangement options were investigated.
Speed was limited to 25knots to keep the size of the NSRP down. Nuclear was chosen for the reasons erik_t noted, as well as evasion and cost (AIP is probably more expensive, I say "probably" because (a) AIP submarines are used for different roles so they are not directly comparable, (b) published submarine costs are suspect as they are "expressions of national will" (especially for export as they can be sold at a loss or subsidised), (c) there aren't any German SSNs or British AIP SSKs so we don't know how much of the cost differential is due to other aspects of the boat). Hybrid propulsion systems with downsized nuclear plants and large batteries/fuel cells continue to crop up in studies at UCL and elsewhere as a method of stopping UK SSNs ending up spherical.
[Edit] The other reason for using an SSN as a basis is endurance. Below some endurance level it will be better to carry big tanks of H2 and O2 rather than renew it. Using current storage technologies, this design was way past that endurance level.
RP1
NB// Drawing changed to correct my borked mast copy-paste.