It would might have been less rusty if stored undernwater.I've seen less rusty Alfa Romeos from the 1970s. Where in the garden was it stored - the pond?
Food for thought....Why does someone treat a relatively expensive item like this with such disregard? Statute of limitations in England/UK ?
What’s really sad is that it’s had very use, still on its original tyres which are like new.
I could be interested in 'improving' Bromptons if I could buy a genuine Brompton frame and add parts of my choice. Choosing wheels, gear etc would improve the experience for me. As an example I recently made the move to 160 mm cranks and fitted a front derailleur - minimal weight penalty, but more 'my' bike.