HMS Trent outbound from the Clyde on sea trials
Beautiful ship she is. I also cannot "fantasy" about up-arming her. But, at least if T31e program goes on, there is no need to up-arm River B2. In this case, they can be ships among the highest grade of EEZ patrol ship, lightly armed, but with CMS and network, so-so high level and with very long range. River B2 has accommodation up to 60 or 70 crew, in addition to room for 50 RM troops. Huge margin.
But, if "more T26 movement" is executed and T31e be hollowed out like Floreal-class (with 1 more T26) or even banned (with 2 more T26), River B2 can be a good candidate to supplement the "(6+9 =) 15 or (6+10 =) 16" top tier escorts (T45 and T23/26). So, River B2 is a good "reserver" for the future uncertainty of T31e.
---- a bit detailed ----
River B2 is nearly half in size compared to even Leander, so "up-armed River B2" can never be a light frigate. But a 90m 2000t vessel can go anywhere. Then what to cut, for me, is helicopter capabilities. If the ship operates near Britain island, Falkland island, or other territories with air-force/naval air-base, land-based air cover is much more efficient than helo onboard. (Actually, many OPV for near-shore patrol lacks hangar, or even a flight-deck).
On the other hand, if the ship is escorting RFA vessels, all of them (now) has a good helicopter facilities
and River B2 do not need to carry helo by their own.
There looks like there are some volume of niche here.
In this case, we do not need to think of hangar anymore, and utilize the whole "backyards" of River B2 for her armaments. They can carry
2 more USVs, some SSMs, or even 3-cell ExLS for 12 CAMM. Also they can replace the forward gun with 57 mm Mk3 with 3P and Orca rounds (*1). At least, the 57 mm turret itself is not that expensive.
*1: Three 57 mm guns can be purchased with only 18M GBP (6M GBP each)
. Of course, we need the whole logistic and training to be added. But, at least the former can be "shared" with USN and Canada. Adding Orca round could be expensive, but we can regard it as an option. (https://ukdefencejournal.org.uk/bae-to- ... 2-7m-deal/