Didn't TV Centre have permanent facilities for visiting international commentators at one time? Probably inadequate for the Euros and the number of broadcasters who would want to attend. I know on occasion they built facilities on studio floors, particularly for the Eurovision that was held at the Centre.
Not sure what you mean by 'permanent' - but yes, multiple commentary areas for off-tube, off-site commentary were sometimes built in studios at TVC when the UK hosted large events.
These days any major event like a World Cup, European Championships, Olympics, Paralympics etc. would need an IBC to be built. It would be close to impossible to accommodate the requirements within existing studio facilities - however if there was suitable land near an existing broadcast centre in which to build an IBC then that might make sense. Similarly if a broadcaster wanted to move to new facilities AFTER the event, and take over the IBC as a new venue, that would make a lot of sense.
However IBCs provide core infrastructure (feed routing, recording, managament etc.) but individual broadcasters who hire space in the IBC usually handle their own equipment provision and installation, via third party systems integrators (if they don't do it in-house) So the facilities used by the BBC, NBC etc. in Rio won't have been part of the IBC spec, they will have been added by the BBC or NBC, and removed by them at the end of the event (BBC did a sharing deal with C4 to share galleries I believe, so C4 are now using the BBC galleries in Rio)
There were discussions about the BBC or ITV taking over the Stratford IBC after London 2012. In the event, Timeline/BT Sport took over some of the space, though as discussed above, this was largely an infrastructure benefit, they weren't buying fully equipped galleries that had been sitting empty.