I recently applied for a job and didn't get shortlisted. I was told that I met the bar for interview, but so did another person who scored lower than I did on the shortlisting, however because he ticked that box I got bumped off the interview list, which didn't really feel fair as I felt discriminated against for not having disclosed any issue. I felt that they should interview us both, but hey ho, not a lot I can do.
Firstly, you can't be discriminated against if you didn't disclose your disability (in this case didn't tick the Disability Confident box). Discrimination can only happen if they know you had a disability and deliberately excluded you.
Secondly, what you describe isn't how Disability Confident works. All candidates have to meet the essential criteria for the vacancy, whether you have a disability of not. It would not be in the recruiter's best interests to bump you off the list just because someone else ticked the box and you didn't. No way would they throw away the possibility of the right person for the job, just because there was also a DC candidate in the sift as well. They would interview you both.
They are being disingenuous, their limit of how many candidates they choose to interview is self-imposed and should have said "you didn't meet the essential criteria on your CV so you were sifted out".
It's entirely down to you whether you elect to tick the DC box. With your disability, it sounds like it's impactful enough to disclose it immediately so you might as well use the scheme to get your foot in the door and do well at interview. Unless they mention it at interview, you have no need or obligation to discuss it, you would be better off focussing on impressing them with your skills and ability to do the job.
If you're asked the question "tell us about your disability" they are breaking the law, just as if they asked you to tell them about your childcare arrangements, they are unlikely to ever ask a man that question. If they do ask you, you have rather a good indication of how supportive, or not, they are. They could get round it by asking you whether you need any reasonable adjustments for the interview, which isn't breaking the law. If you don't think you'll need to use the toilet for the 1 hour-ish interview, you could just say no thanks and they shouldn't take the conversation further.
You are not cheating or being dishonest by ticking the box. You would be if you didn't have a disability and ticked it to get an interview. As you have a disability you legally have a choice under EqA2010, it's all about dignity and choice and putting you on a level playing field with everyone else. Good luck!