Updated – 7/1/17 (refer end of post)

My Brompton bidon cage system is still my favourite but it doesn’t suit someone that wants to use their folded Brompton as a shopping trolley, with handle bars unfolded & a bag on the luggage block – because the cage would foul the bag & regularly removing/installing the cage is best avoided. (Whoops, see update below.) A cage that is readily removable is the Monkii system but after trying one for some time, I’m not keen on 2 aspects of the design. First, I don’t like strapping the bottle to the cage & having to remove the two together when drinking. The killer issue though, is that the Monkii cage can readily be ejected when riding over bumps. So, here’s my attempt at fixing those 2 issues…
Peregrine with the old system… 

 
… & Clarence with the new setup… 

 Here’s a pic of the old & new components… 

 
Recipe: take one Monkii system, remove the strap & cut off the wings & tail of the cage. Drill & countersink the cage spine & attach to a standard cage with some screws & nuts. When the Monkii stem block is being mounted, hook an O-ring under the block. When installing the new “combo cage” to the bike, hook the top of the cage under the O-ring & stretch it up so that the Monkii portion of the cage can be connected to the Monkii stem block. 

 
So, I now have a Monkii system that lets me easily remove/attach the bottle when drinking & won’t become disconnected over bumps – & lets me readily remove the cage when operating in “shopping trolley mode”!

Update: Since using Ralph-e & trying to avoid removing the luggage bag (& disconnecting the battery), I now realise that I can leave my Bidon in the cage system & the bag just pushes the Bidon to the side (as the bag pivots around while folding the front wheel).