Firstly, River B2 (my estimate 83M GBP) and T26 (my estimate 730M GBP) is 9 times different. So, there is a huge gap between these two. Something "in between" includes many of the exiting escorts.
Secondly, 6 for 2B GBP means, if new design, 2000/(6+2 [design]) = 250M GBP/unit. In cost difference, it is 1/3 of T26 and 3 times that of River B2. So, it will be right in the middle of T26 and River B2
. (In other words, confusing River B2 with a 250M GBP T31 is similar to confusing 250M BGP T31 with T26).
If we have existing (= already built) design and just modify it, the unit cost will be 2000/(6+1) = 285M GBP. This means, we shall go with Khareef mod (Cutlass) or River B2 (Avenger) in BAE case, and mod Samuel Beckett (VARD7 90), or (cooperate with Eastern Shipbuilding and) modify the Offshore Patrol Cutter (VARD7 100 significantly improved) in Babcock case.
If ASW is the top priority, this T31 is not a good choice. But, if we can let T26 concentrate on ASW while T31 taking all the other tasks, maybe T31 will become "good", hum..., "so so good". All will depend of how "other tasks" will be needed. At least in this decade, tasks other than ASW was the main part of the RN job.
< personal opinion
Among them, OPC is actually very large vessel. 110m long, 16.5m wide, 10,000nm range and 60 days endurance (see https://www.uscg.mil/hq/cg9/opc/pdf/OPC.pdf
). May be as large as 3500-4000t FL. May be we shall reduce its range to 7000nm and endurance to 35days to make space for additional armament to be carried.
With this hull, it can do all of the T23's task, other than ASW. I guess. But, it is expensive, as well. May be we can omit 127mm gun and start with a ship with 1x 57mm gun/CIWS
, 24 CAMM with 2 datalink, Merlin-capable flight-deck and hangar (will only carry a Wildcat and 1-2 UAV, never a Merlin, as you know), small hull-sonar, torpedo-defence system, decoy launcher with ESM. At the stern, may be able to carry something (USV or ORC), as well.
As a "FFBNW", we can add 127mm gun (in place of 57mm), and 8x SSMs. Interestingly, the propulsion is DODADE, with electric propulsion up to 6 kts (at least in the original Vard7 100 design). If we can improve it to 10 kts, and add CAPATAS2 or so (of course as FFBNW, when built), it will make a "so so good" shallow water ASW vessel.