I'm trying hard not to answer my own question and let the more cerebral Pruners make some points but we should at least acknowledge that a new system (either missile or submarine or both) is inevitably on the list of jolly expensive things to procure in the next few years and it will have an extremely distorting effect on the rest of the equipment programme.
So how do we get the best/cost effective solution? Single role, submerged delivery with a global reach is clearly one way. How about a nuke option for TLAM and then multi-platform it - T45 and ASTUTE for example or at the risk of exciting the BLUE STEEL club how about a Stormshadow launched from a Nimrod MRA4 - good reach when the mother is aerial refuelled even if speed of response might be a bit more sluggish (assuming the submerged bomber is in the right ocean). Even more futuristic are some of the things that were thought about for FOAS - some sort of cruise missile deployed from the ramp of an A400M that powers up as it falls away from mother. Come on chaps lets have some original, but sensible, thought!