"The information I've gleaned about 'the Great NHS Data Grab' does not explain to me what the data can or cannot be used for"
Anything at all.
"who will have access"
Anyone who pays for it. Shortly thereafter, anyone with access to the internet.
"how effective any pseudonimisation may be"
Not at all.
"or what benefits the NHS will obtain from allowing access to commercial organisations or other country's health organisations."