Problem is as soon as they are uncovered they will be picked clean.
Monkeys, like elephants, can never have nice things!
I wonder if pines or palms would work? Inedible, not geographically accurate but would provide greenery and flourish in the UK, especially palms in the warmer inner London climes. Over time, pines would provide a thick layer of dry needles for scatter feeds on the ground.
A few fast growing leylandi would bulk the enclosure out and provide good climbing places.
Or Monkey Puzzle Trees-my Mum & Dad had one that grew from 3ft to 30 ft in only a few years at their old house.
Maybe even some fibre glass trees rather than a climbing frame?