As I proposed in the escort thread, I think RN shall better stop pretending "19 escort" saga even though the actual sea-going days shows virtually only 12 is active, and shall cancel 5 T31 to get 1 more T26 and up-arm T45 and other assets.
Along this line, it will be River B2s to carry out most of (say, two-thirds of ?) the tasks currently assumed to the 5 T31e.
Fortunately, RN are going to have 5 River B2 very soon. At the same time, keeping 3 (or 4) River B1 for EEZ/fishery patrol is technically very easy. Then, up-arming River B2 will be of great importance if RN can cancel 5 T31e. What is an option here?
1: Apart from many "high-grade" up-arming plans, just adding the LMM launcher to the forward 30mm gun is a very good starter. For "more", adding two 30 mm +LMM turrets on their bridge-wings will also be a very practical approach. With commonality, additional naval engineers will be limited. With 360 degree coverage, (supported with 12.7mm and 7.62mm guns), it will make River B2 a good "close-in fighter" for fast-boat attacks.
2: Another option is, of course, adding a 20mm CIWS or two. But, if you look at the system's photo, it is evident that Phalanx is much more complex than a 30mm turret, and maintenance load needed is much more higher. But, Phalanx can shoot down simple ASMs, as those operated by Houthi-rebels or even by Hizballah. No, Phalanx is not good against super-sonic ASMs, but super-sonic ASM is very large and costy, and enemies operating it is limited. So, adding 20mm CIWS is also a good option, depending on the theater those ships will be sent.
3: Because land-based air-cover can be expected in many cases, lack of helicopter can be accepted. But, adding a UAV will also be a very good option.
4: Other key assets will be, LRAD (sonic weapon), strong search lights, smoke grenades, powerful water nozzles, RHIB-based patrol USVs, and underwater frogman detection high-frequency sonar. Note these assets are not "high-end" expensive weapons, but is very useful in anti-terrorists patrol, I guess.
These 4 options are pretty much "laser focused" on anti-terrorism, or in-quasi-war situations, I think this is very realistic/practical "up-arming" of River B2.
With LMM becoming reality, those "modestly up-armed" River B2 is more and more "easy to imagine" now