"what's the cheapest way of delivering credible minimum deterrence (so that we can spend the saved cash on the conventional forces)?"
First off, your exam question is wrong. It is important to understand what effective deterrence is.
Cheap may get you a "we can claim we have deterrence" but it may not give you what you are trying to achieve.
So, what is the most cost efficient EFFECTIVE deterrence. To be effective, it must be credible.
Deterrence is by its nature a DEEP fight. The TACAIR answer is risible. "Stealth" is as much a marketing ploy as capability, which erodes over time in its ability to "try not to be seen."
The TLAM-N deterrent model might be viable against mid to low tech enemies. For example, if you were a mid to low level power and had as your deterrent object a mid to low level nation, you might get away with X number of diesel subs able to carry X dozen TLAM-N equivalents.
Against any significant EW/IAD architecture that deterrent doesn't fly. (Pun intended)
As this is a Brit topic, I'll back out now. I am also a few years beyond being involved in such theater strategic level issues to be current in my thinking.
It is interesting to watch the discussion.
Any number of mid sized powers are keen to establish a deterrent capability: Iran, for one. I daresay they want a legitimate deterrent, not a paper tiger deterrent. For that matter, the NorKor goons are striving mightily to establish a credible deterrent that goes beyond the local theater in reach.