People will say it wasn't a con, and they're right.
But people will also say you were daft and shouldn't have handed money over: that's true but it's a learning experience and you won't be as easily led next time I'm sure. See it as an expensive lesson, £100 is a lot of money but it could have been worse.
Remember next time, you don't have to pay money for anything you don't want to. The following phrases are fine:
I'll have a think about it and come back if I decide to buy.
I don't actually have my wallet or card on me, silly me!
I'm not interested, thank you.
I've already said no, you're not listening to me so I'm going to leave.
You absolutely don't have to buy something OP, they can't make you, and if they forcibly grabbed your purse and swiped your card you've been mugged which is a crime. But plenty of people fall for this stuff out of feeling awkward finding a way to politely decline. The key is to let go of trying to be polite, a hard sell certainly isn't in any way polite or respectful of you even though it's a tried and tested sales technique. So you can be brusque or rude in return, they're relying on you feeling put on the spot and paying because it's less hassle.
I guarantee a lot of people have done this!