Your first photo shows the joist running across from the external wall. Have you felt under the bottom edge of it (I think water may be tracking along it)? Is there another joist, behind that one, nearer the internal wall - have you felt/studied that (with a torch and a hand mirror) for something similar?
Is that outside face of the wall exposed to the prevailing wind and what rainwater goods are above/near it?
Is the house of some architectural merit or listed? Is it in a National Park, AONB or Conservation Area? (if you take remedial action, you may need permission first) It looks like it was pointed with cement-based mortar at some stage, although the wall will probably have been built with lime mortar (you could look at the mortar between the bricks from the inside to confirm this). Cement-based pointing, combined with soft and relatively porous old bricks is not a great combination because moisture can get trapped (and it stops the bricks drying out so easily, increasing the risk that they get so saturated with water that they act as a conduit for windblown rain - as I was saying before).
You could render it (with or without a coat of masonry paint), you could simply paint it with (microprous) masonry paint
https://www.screwfix.com/p/sandtex-ultr ... 0ltr/76225 , you could also possibly use a clear sealant like this:
https://www.screwfix.com/p/no-nonsense- ... 5ltr/57474
OR you could use something a bit more eco-friendly (and more appropriate to a nice old house maybe) like limewash:
https://www.littlegreene.com/paint/finish/limewash
BUT... you need to be sure of the root cause(s) of the problem first and - second - sure that you're not creating worse problems for the future (by using new materials/finishes that are incompatible with the existing).
