Whopity, as you well know from your CAA days, the Authority accepted the NPPL P&SC requirement that training for the NPPL (SSEA) had to be conducted at a training organisation in the UK or IoM. See CAP804 - and yes, I know that you'll bleat about it not being a legal document...
Part of the whole raison d'être for the NPPL was to reinvigorate the
UK training scene, as well as reducing costs for student pilots due to the shorter, simpler syllabus.
So no, 'car boot' instruction is not acceptable for the NPPL.