I wouldn't be so sure, from the 2009 Hansard written answers (looking at Wiki there apparently is a figure in thee 24th Nov 2010 written answers, but upon further investigation there isn't anything) the total running cost for the Type 23s, the whole lot, is £340mn.
This makes a 'unit running cost' something closer to £26mn per vessel, if you can get 3 River class OPVs for that money - something which I doubt, the MOD payed £39mn plus the fees even after they were a decade old so brand new ones would probably be more expensive then factor in the extended construction period would more than match the inflation rise of the T23 costs IMO - then I don't see the point in laying up a T23, that sort of money is small change to the MOD it'd be barely noticable.
House of Commons Hansard Written Answers for 09 Sep 2009 (pt 0024)
EDIT: These are the parameters for calculating this 2010 figure
EDIT #2: Not to mention then it'd be a VERY short and easy step for the Govt to say "We don't need this extra frigate at ER as it's getting no use" and decommission the ship completely and knock off a T26 or 2. Not my ideal scenario, but I
might accept it if it was just the 1 reduction
maybe.
In the ideal scenario we'd keep the escorts and procure a couple of OPVs, something which - looking at the River figures - wouldn't be too impossible to imagine happening.
This was something I thought about the other day, if the first T26 is coming in 2021 then - assuming the idea of 1 being delivered per year - then the last T23 will be bowing out in 2034. If it's St Albans that'll mean 32 years service, not bad.