Card shouldn't matter as Mrs and Old Man have brought stuff for me with my License presented.
When you say the Heathrow shop do you mean thy said yes to the Blue badge or no to it?
Sometimes with shops its an Assistant taught one thing where a manager may allow another, Gunshop may be worth chatting to as he's going to be the most clued up of any of us being a RFD.
As to Journey it depends on how it feels to you, if your Comfortable and can maybe make a day of it then the risk is lower but if you're going to suffer discomfort to do it I'd say maybe give it a miss.