This has surprised me somewhat and I'm not sure how true it can be.
Looking at your examples, the ERCMAC webpage looks like it was built many moons ago, and refers to support by 'Anglia Television' (along with an @angliatv.co.uk email address!) and a Bafta nomination for a film they produced 'this year'. The short film referenced was nominated in 1999! I would therefore guess that this is a very old webpage and wouldn't think that there is any such company based at Anglia House.
Likewise, the British Film Council page posted is for a production from 2003, with Anglia House indeed referenced as the address for a company called ScreenEast. Their current website looks fairly modern (albeit using a screencap from Sky News circa-2003 on the homepage!), but lists an address in Chesterfield as the contact address now, so again they may have been at Anglia House many years ago, but aren't now.
The White Label Media website, however, is more interesting, as they are clearly an active operation and do list Anglia House as their address (both on their website and in their latest set of accounts submitted to Companies House). They also state that they are 'BBC and ITV approved suppliers of production resources'.
However, the Anglia House site is full of oddities so I'm not sure it's as clear that they rent space in Anglia House as it seems. In the 1980s/1990s, on the Anglia Square site Anglia TV occupied three buildings, Anglia Square, Townshend House, and the Old Post Office. There are upper level walkways still between Anglia House and Townshend House, Townshend House and the Old Post Office, and a completely out of keeping structure that fills the gap between the frontages of Anglia House and the Old Post Office with meeting rooms and offices. Until recently the Old Post Office was an Estate agent, and as mentioned before Townshend House has been rentable office space for years (although there's still a big satellite dish out front which is presumably for ITV!). I therefore wonder if the ITV connection or if they occupy a bit of space that connects on or into Anglia House means they've used the address for telly heritage? The only clear entrance to Anglia House is the main ITV entrance and goes almost directly into the newsroom, so unless there's another entrance by the loading bay or something (which would seem slightly strange) I think ITV remain the sole occupants of Anglia House. Irrespective of whether they are or not, it's still a smaller space than Yorkshire have etc, but it would put them in a relatively unique position of being in their original building and not in shared offices etc?
Alternatively, you might have a much better knowledge of the whole thing than me and I could be entirely wrong!
623058: it just seems like your an mp3 whore