You'd need to point it out to them, and probably get them to sign it too. It'd hold up in court if you had a solid cancellation policy.
The problem is though, your reputation could be tarnished if you kept suing people for money in your area.
What I'd be working on more so would be ways to stop you losing out if they did cancel. And then work on getting the % of cancellations down year on year. And just swallow any small loss that year given the fact your reputation would still be fine in years to come I'm sure you'd not actually lose out on anything during your tiling career.
Don't fall into the conservatory company type affair that are always upsetting customers when the customer cancels because they have to outlay so much you have to have good cancellation terms.
And to be honest, a job less than a grand I don't think I'd be too upset after I'd posted a nice big rant on here and cooled off. A job more than that and I'd try to speak to them at the stage between you handing your quote in, and the job start date, quite a few times. Then they have a few chances to cancel.
As for a cancellation via text, I'd just not accept it and turn up and make them feel fools. But I wouldn't count on money or getting their custom in the future.
