Declining intervention is definitely an option hun. I think sometimes the 'professionals' forget this.

I have no idea about birth centre policy but I think if you're uncomfortable with it, then don't turn up to the appointment. You could call ahead and inform them you're not coming (to be polite) and double check it won't affect your ability to birth there. I was pushed into an induction and it was booked but I was so uncomfortable with it I rang my doc back and told him why I didn't want it done. We negotiated more time and I was happy with that and went into labour the next day after the induction was supposed to happen anyway.

Given your history I can't think of any reason why they'd be in such a hurry to S&S you at 40 +2.

There's two full days before Tuesday so hopefully it will become a non-issue but I would be doing the same as you (avoiding it) and taking the attitude that they are the ones that should justify to you why it's such a good idea (not you having to justify to them why you don't want it).

Good luck hun.