I honestly cannot believe people are saying to go in a wheelchair.
What do you plan to do if they ask to see your disability badge?
You have - in your own words - the beginning of SPD. That is absolutely no justification to pretend that you have an actual disability, and is frankly a huge insult to people who genuinely need wheelchairs, you know, in order to move. Yes, the festival people are being wankers, but unless you can prove that you are physically incapable of standing, then they aren't obliged to give you allowances.
You can take a chair, or stool and risk it being confiscated. You can make a decision about whether or not you are physically comfortable to go to the festival bearing in mind the rules, and either work with the rules (soft cushions, etc) or, you can decide not to go.
What you most likely COULD do, is get a full refund for your ticket if these rules were not established at the time of point of sale.
People are throwing around the equality act as if it's some kind of free for all that has to take all inconveniences, whether temporary or not, into account. My sister has epilepsy - she can ask festival organisers in advance if there will be strobe lighting, but she can't stamp her foot and demand she gets to be whisked off backstage when it's in use, or demand that it isn't used at all.