Filling a cavity with anything is a breach of the UK Building Regulations.
Where does that come from?
Also, the extent of rain penetration to an external wall has less to do with exposed conditions, but more to do with the quality of construction, the absorption capacity of the bricks and quality of the pointing.
Wind pressure is not a factor, but duration and severity of driven rain is.
Interestingly, a more absorptive brick is less likely to experience rain penetration than a dense brick,so again its less to do with the actual weather
Rainwater will tend to run down the rear face of the external skin once saturated. Whilst it can go horizontally if a route allows, gravity, time, evaporation, heat (from the inside) and suction (from the outside) are all acting against it.
The health issues and allergic reactions to urea-formaldehyde foam should not be underestimated. Once this is used, if the occupier does suffer a reaction, then the only recourse is to move out
Basically, there are few issues with retrospective cavity insulation. But it may highlight and heighten existing problems with the wall construction
Many installation companies will try and make up for a poor survey with a myriad of get out clauses within the contract