So all in all they resolved your complaint within 48hrs and to your satisfaction, that seems quite reasonable, to the point it almost makes your initial reaction look the opposite.
Bitter experience dealing with this type of company and, while I've had the outcome I wanted will be a bit of a doubting Thomas until it's confirmed in writing or my bank details show the correct outgoing.
So no, I don't think I over-reacted to the situation where a company disregarded the contract they had agreed to put in place then wanted to charge £16 a month more than even the full price was worth...