Well, I’ve definitely got a working card right here. I’m sure other Mondonauts here can also attest to this as well as those on Twitter.
As discussed in this thread, the £100 is an initial top-up that you can spend as soon as you receive your card. It helps ensure that cards get actively used and don’t end up in drawers during the Beta period.
Feel free to browse around the forums here, ask questions and decide for yourself though. I believe your invite will remain active indefinitely.