Why not just look in the room above and see how the floor boards/sheets are screwed down?
If the joists do not induce any loading on the wall then all the beam the has to take is the weight of 15 of those blocks plus weight of plaster (assuming its a single skin wall)
Middle route is calculate the load the beam will need to take and then calculate what the bending moment will be and select a suitable sized 2m long UB from https://www.steelforlifebluebook.co.uk/ub/ec3-ukna/buckling-resistance-s355
All as clause 4.14 Doc C building regs https://assets.publishing.service.gov.uk/media/5a8192a0e5274a2e8ab54b5f/BR_PDF_AD_C_2013.pdf
Don't need them on all walls just opposite ones to provide cross ventilation and avoid short circuiting
Working on a roof load of 2kn/m2 a 6m joist bending moment would be 27knm. Looking at Blue Book a 203 x 102 x 23 would seem to suffice (subject to a shear and deflection check) Perhaps Tony1851 can concur
Throwing up 9 screens of info in one go, one screen on my laptop is my scope, you must have a viewing screen as big as NASA (or very very good eyesight)
I have never seen a lintel/beam collapse either, I reckon that's a builders myth as well but you know how they like to exaggerate, and as for the drainage jetters what a complete con