While I can see it being totally wrong to post on YouTube, I'm not convinced that TYITC would be correct about the Data Protection Act. The DPA concerns personal information, which is held by the organisation (such as staff details, customer details, where there's more than just a 'distribution list', so for example some sort of payment record for a customer).
The individual who is subject of this video is not necessarily a customer of the estate agent, and there's no proof they have any details (not even her name) on record so it would be a wild goose chase to involve the Information Commissioner, and a waste of time when there are serious data misuse issues to investigate.
I'd expect someone in the local Council's Business Development section (which, after all, has a wish for businesses in the town/area to be held up as good examples) and/or staff at the Chamber of Commerce, to be likely bodies that would accept a complaint and make strong representations to the firm.
Do go down the Trading Standards route first, asking them who deals with the DDA, and whether a business is breaking any rules on posting such a vile video clip online.
Also, if you can, see if you can download a copy. There are a few add-ons for browsers like FireFox, which can do this (I think), so you have a copy as proof (and take a screen shot of the browser - use ALT+PrtScreen and then Paste in image editing software like MS Paint (on the Accessories menu on Windows).
It's worth having copies that you can write to a CD, if you want, so you could anonymously submit a report to the local Police (put some suitable description like "YouTube video clip humiliating a person with mental health problems" on the envelope, and they may take it from there).