The siamang enclosure at Monkeyworld is, in my mind, a benchmark for outside space in a temperate zoo, admittedly thought I've not seen this in person:
If the newer gibbon facilities at Monkeyworld have anything like the quality of outdoor habitat that the siamangs have, I'd say these would be well and above best practice for this species in climate typical of the UK.
The Siamang enclosure at Monkeyworld is good- the exact opposite of Marwell's. The only drawback IMO is that the trees are mature Scots Pine- not very natural and I wonder if they get sticky pine resin on themselves a lot as a result- but the whole Monkeyworld site is Pine/heathland so there is no choice of other tree-type.
The last time I went(some years back) I noticed the range of other Gibbon enclosures, although not overly tall or large, were long and very well equipped with carefully-placed hosing etc to allow maximum scope for brachiation- again the complete opposite of Marwell's Siamangs.
What I'm a little unclear about is whether there is/are now more open-air enclosures for more of the Gibbons, apart from the longer-established Siamang one.