Ellie
The stuff I pointed out is not original Brompton, but the cheaper Chinese copy, but the parts are all compatable.
The QR block would only require two small holes to be drilled and threaded.
One cautionary note :-
If your handlebar stem angles forward when unfolded then the bars would foul the bag when you turn the handlebars (as I found out when I tried to fit my Brompton bag to my GoBike)
I can't tell from your folded photo if this would be the case for sure, but it does look as though you could easily fit this replica block and waterproof bag.
The black QR block can clearly be seen in this
photo