Many on here will tell you that you should confirm on the morning of the booking.
My guess is that as she hadn't heard from you, she thought that it didn't look like you were going to turn-up, hence the text from her 30 mins before the booking, although you told her you were still on for the punt, I think by then she had already booked someone else in. Touring is costly. If she's getting a lot of enquires, and has not heard from you even though there was just 30 mins to go before your booking, and you hadn't replied to her email, the temptation to book another punter in would be very strong.
If she hadn't sent a text, 30 min before the booking, I presume you would have left until 15-20 mins before he start time, before texting her. IMO, you really need to contact her a lot earlier to see if the punt is still on, all the more so, if you've had a fairly long journey.