In fairness to the OP, I'd be pissed off too if the waitress said there was a 12.50 min charge but didn't indicate it was per person & I ended up paying $80US for 2 teas & a coffee.
I've sat at tables that have small placards & don't always read them, as often they're just hawking some type of meal, drink, etc, so it might be easy to overlook especially if the OP was in the middle of a biz meeting.
The OP posted on TripAdvisor and has posted here, so I think he's done his share in giving a heads up to others so they don't accidentally encounter what he did. I, at least, appreciate that.
To the OP - chalk it up to unfortunate experience & move on at this point. Hopefully you were staying at the property so you'd get points for the incidental spend.
PS - while the StP usually gets good reviews, this is certainly a negative for the property. I've not stayed at any other London Marriott properties that have a minimum spend.
Cheers.