I agree, I don't think you need anything - the only time people are being challenged around here is public transport heading into London. If you don't want to/can't wear one then don't! It really is that simple.
The hidden disabilities scheme is amazing and I agree with Stella there are a lot of people who are now picking up on it and abusing it because they don't need it, they are using it as an exemption and because you don't need any id or "proof" for it anyone can get one and it is watering down the impact and what it actually stands for...
