Maybe this article from Richard Beedall will be of interest?
Vanguard SSBN Replacement
In order to keep costs down, a new submarine design has become unlikely for a Vanguard replacement and current thinking assumes an evolution of the Astute design - indeed BAE Systems Submarines has already examined two variants fitted with an extra hull section. The first includes the fitting external to the pressure hull of sixteen Mark 36 Vertical Launch System tubes for missiles such as Tomahawk, and the second includes four Trident II size (86 inch diameter, 36-feet usable length) missile tubes, installed aft of the fin. The later approach is preferred as the large tubes are extremely versatile, alternative to Trident II SLBM’s they could potentially carry a next generation ballistic missile, a multiple all-up round canister accommodating seven Tomahawk cruise missiles per tube, equipment and swimmer vehicles for special forces, Unmanned Underwater Vehicle’s (UUV’s), deployable decoys and sensors, and even encapsulated Unmanned Air Vehicle’s (UAV’s). While a re-role will not be trivial, the new submarines would certainly be far more flexible than the current SSBN/SSN divide permits.