Chewbacca, you have a number of options. You can walk away knowing that you have dodged a bullet. You can post your experience on a wider social media site, as you have done here. But if you are up for some confrontation you can get out your cudgels.
First there's the question of proof. Was the £389 + VAT estimate in writing? If not, proof of that will become an issue. If it was in writing you can post a copy along with the copy of your subsequent invoice to the company's head office, asking for an explanation. Enclose a stamped address envelope, don't use e-mail, they get ignored.
If you get no satisfaction there, you could send all the details of your experience to a newspaper, either national or local. They will love championing your cause, if they think that you have a good case.
Finally, although you haven't been ripped off, thanks in part to your son's intervention, the company did engage in a non competitive practice, and that's illegal. The Office of Fair Trading will either advise you, or if they think it's serious enough, investigate it on your behalf.
It's so sad that you've had such an experience and I hope that by now the red mist has lifted somewhat. Perish the thought that I would make you start to see red again, but do be cautious about accusing them of discrimination. Chances are that you weren't seen as an older person, or even female, my guess is that you were judged to be a mug from whom a fast buck could be made. You have a very strong case when it come to anti-competitive issues.
Good luck, let us know what you do.