But I'm not convinced that using some freeware web site or software for weather or NOTAMs would count as due diligence, so I go to the Met Office and AIS web sites.
It's an interesting point.
It's hard to know where the UK CAA stands on this kind of stuff (third party notam sites which attempt lat/long parsing and a graphical presentation, and the countless weather websites which nearly all present data from GFS).
They are still mostly denying the existence of the internet - except that preflight briefing is
de facto now impossible without it. They certainly avoid any mention of GFS; all the PPL training stuff is TMK purely UK MO material (F214, F215, MSLP).
There is now a totally bizzare gulf between the material (and the environment) presented to a pilot during training, and the material (and the environment) which the pilot will be working once flying for real.
There are no CAA prosecutions I know of in this area (what constitutes due diligence for preflight) and I am damn sure they would be extremely reluctant to try one, because given the PPL training syllabus a defence lawyer with more than one braincell would wipe the floor with them.