You can choose to tell them at any point. They aren't even meant to ask on the application form any more whether you have a disability or not - so ticking a box isn't applicable unless the employer is wanting to discriminate against people with disabilities...
Which is illegal.
And yes, you CAN ask at any point of your employment, whether you have been there for 2 weeks, 2 years or 20 years for 'reasonable adjustments' to be made in respect of your disability, EVEN if that disability wasn't declared at any point previously.
In practice, I would advise trying to get by as much as possible until you are out of your probationary period, and requesting the reasonable adjustments once you have protected employment - but even if you DO ask whilst you are still in your probationary period, if that company then 'lets you go' at the end of that employment period, without good reason (which means NOT connected to your disability), then that company needs to be VERY careful - if they then hire someone else to do your job, you have a claim against that company for disability discrimination.
Because it WOULD be discrimination to not keep someone on after their probationary period just because they needed reasonable adjustments due to their disability if there were no other good reasons for letting them go and you subsequently have that job to someone else.
It would be astonishingly easy to prove that you were discriminated against on the basis of your disability if there were no reasons for not being kept on, the job was still there, and you had hired someone else to do the same job...
It's WHY the Equality Act 2010 tries to prevent this.