Makes you wish we had more ships in reserve. Williamson was looking into keeping some of the Type 23s when the Type 26s arrive. Far chance of that now.
There are lots of problems with keeping ships in reserve which is why, AFAIK, no one still does it, or does it credibly (rusting away waiting for a trip to the breakers doesn't count)
1. Ships degrade relatively fast in a salt water environment and our weather. Yes, you can coccoon them, but there is much more to keeping their systems protected than simply applying a protective coating etc. People need to maintain ships in reserve and that costs money..
2. They need to be updated to remain relevant. Furthermore the systems on the ship still need to be supportable. A Type 23 in reserve would presumably still have Seawolf, for example.
3. We would need to have trained crews for them - no this would not be a problem in a very protracted conflict,
4. In the event of a hot war, the locations of our reserve ships would be known in advance and they would be easy targets for pretty much anything.
5. Most of all though, I'm trying to come up with a scenario in my mind that would need some kind of delayed surge into service of escorts from reserve. I can't think of anything and I've thought this through on many occasions. The last time we did this was in 1982 with (at least) some Tribals, but that was a very different world and the Tribals, whilst built for colonial policing, were arguably not the best choice for it, even then. HMS Duncan cost about £1.050 Bn. For what we need to do in the Gulf, something more like the Netherlands' Holland class OPV at £150M a pop would appear to make mores sense. Something a tad better armed than the OPVs we already have with the provision to embark at least one Jet Ranger class helo and some drones.
1. RFAs have recently been laid up (effectively in "reserve" for periods up to about 18-24 months) with crews of around 15 professional seafarers to run and maintain the systems, augmented with a few shoreside contractors plus cleaners and security guards. Warm layup, rather than cold, with ship's staff in attendance every weekday, electrical systems live, etc. Nothing like the weapons and combat systems of a warship, but a lot more real estate, and with other systems like lifts, cranes, refrigerated holds, etc. These are ships that are already up to 40 years old and have experienced similar periods of layup in the past (including in some cases the unmanned dehumidified cocoon approach). Obviously not zero running cost, but massively cheaper to run than a ship in service (that was the point, plus manpower shortages), and can be available at relatively short notice - weeks or months to reactivate, rather than years to build.
2. As with the RFA example, refit and rotate ships from the reserve fleet into frontline service. Keeps the systems up to date and well used.
3. Plan for use of the Regular Reserve and expand the volunteer reserve if necessary.
4. As would the naval bases and dockyards where they are located and which would be required to sustain the active fleet. Too bad if it's that kind of war.
5. Is your quoted Type 45 unit cost the entire programme cost including R&D divided by the number of ships? Wiki tends to indicate so. The seventh ship would therefore be a bargain as you've now paid for all the R&D and the shipyard has climbed the learning curve to building them efficiently. How to use the reserve once mobilised? Attrition replacements for anything lost in the opening phase of the war ("Oh look, they've wiped out 4 of our 6 active T45s, never mind, we'll just open another bag..."). Or to sustain operations in the recovery phase - surge the six active ships for an operation, knowing that they will need refit and crew stand down afterwards, but that you'll have another 2-3 coming online as the operation winds down, to cover the return to routine tasking. T45 examples, since you mentioned T45s and I'm too lazy to count or think about T23s or fleet submarines or whatever.
A reserve fleet isn't zero cost, but it could be a useful way to achieve scale in wartime, if the war is of the nature where we have a "transition to war" phase. Our land forces count on a transition to war to mobilise.