Assuming we are indeed discussing Signal Point, the office block in front of the station, it's been in and out of use for ages, with various plans that mostly have come to nothing.
Back in 2012 there was a "Swindon Masterplan" that included the re-cladding of it, but that website has now gone offline and the plans, well, who knows. Here's a ref to it at least:
http://www.jonratcliffeblog.com/2012/09/swindons-regeneration-plan-weve-been.html
There was a thread on here about it back in 2012:
http://www.railforums.co.uk/showthread.php?t=70136
Here's another link ref the proposal to become flats:
http://www.thisiswiltshire.co.uk/news/13317678.Signal_Point_tower_could_become_flats/
I remember reading an exploration account from an 'urban explorer' group a few years back. I think it was 28dayslater but I'm not sure and can't find the web page now - it may have been taken down or my memory may have been playing tricks on me! At that stage, IRC, it was in a very sorry state and even had some parts of internal floors collapsed and windows and doors rotten and falling out in many places.
There's a "Swindon 2020" masterplan that promotes an 'arc of opportunity' area of development up to the station. This highlights that signal point is in a sorry state and that it's hardly welcoming, but curiously has no proposals for it. Assuming it's owned by Network Rail, perhaps the plan is hoping to prod NR into action?
http://www.forwardswindon.co.uk/wp-content/uploads/2016/05/Swindon-TCDP-ES-FINAL.pdf
Despite the proposal to turn it into flats, it appears to be available to buy or to rent. Anyone got £507,741?
https://propertylink.estatesgazette.com/property-details/5567461-signal-point-swindon-sn1-1fe