The idea behind the original South Bank plan was to have everyone under one roof and for a company as big as ITV not to have it's own HQ is apparently fairly unusual. What it has now - rented accommodation in Grays Inn Road, Holborn and White City, along with the hired facilities at TVC and BT Sport - probably isn't that conducive to being a long term arrangement, it was only intended to be temporary after all.
I would have thought that ITV will have to find a new HQ at some point, though it wouldn't necessarily have studios in it of course. My prediction is that they'll end up relocating everything under one roof at either Chiswick or Stratford (the latter would provide a river location too should it really be that important)
Stratford would be a strong bet given how many companies has relocated there although I imagine they’d keep the daytime shows at TVC.
The main reason for not returning to the southbank is due to the fact they are very happy and content at Television Centre. They were worried about the move at first, and would the four daytime shows settle in, in their new studios - and this far they have been very successful.
The facilities offered at Television Centre are fantastic, especially with TC3 housing GMB and This Morning. TC3 was always a brilliant studio, and it is helping now.
Their office space at White City is perfect for them, along with the on site production offices near the studios, so all in all, management thought, why move? Five years rent to move back all of this to the southbank, no, let us stay here.
Their current five year deal with BBC Studioworks and Stanhope will no doubt be extended, in the same way the BBC and Elstree Studios deal was extended in 2017, and will no doubt be extended in 2020 when that deal is up for renewal.
Overall, a good move for ITV. Just, not having a proper headquarters is a bit weird. BBC have got Broadcasting House, Channel 4 have retained Horseferry Road, ITV should have some administrative HQ, and Grays Inn Road would suit it well, as ITN is based there, it would be a perfect home.