Astute will already have a larger torpedo room than Virginia (36 weapons vs. 26) and more horizontal torpedo tubes (6 vs. 4), so the net warloads are about the same (36 vs 38). BAE and the MOD have never absolutely ruled out VLS tubes for the later Astute's (ie Batch 2 onwards). The main driver for fitting VLS would have been if it proved impossible to economically adapt the new Tactical Tomahawk LAM for horizontal torpedo tube launch.
Sources High Low
Length: 3 300 feet 9 inches (91.7 m)
Beam: 3 35 feet 5 inches (10.8 m) 34 feet (10.4 m)
Displacement: 3 6300/6800 tons (surfaced/submerged) 5900/6500 tons (surfaced/submerged)
Surface Speed: 0 Unknown
Submerged Speed: 3 32 knots 29 knots
Crew: 2 110 (including 12 officers) 100 (including 12 officers) + space for 12 additional personnel
Armament: 5 or 6 Torpedo tubes for Spearfish torpedoes, RNSH (Royal Navy Sub-Harpoon) anti-ship missiles and Tomahawk land-attack missiles (TLAM) - 36 to 38 weapon capacity.
Decoys and ESM will be fitted.
Fire Control will be provided by the BAE SYSTEMS SMCS system.