1. When it says suitable for under 25 what does it mean? There are some cars that go when you select it. Is the under 25 option a guideline or a rule? 2 What is the excess on insurance? 3. Once you sign up, are they cancellable/changable? Thanks in advance for any help
1. It means the vehicle is suitable for drivers under the age of 25. The vehicles which disappear when you select the option are the ones not suitable for drivers under 25 years. It is a rule regarding the insurance.
2. Excesses below: 
https://www.motability.rsagroup.co.uk/make-a-car-or-wheelchair-accessible-vehicle-claim
N.B The insurer for the scheme changes on 1st September 2023 to Direct Line, but it appears excesses stay the same.
3. If you have a geniune reason (for example, your disability worsens rendering your vehicle unsuitable), Motability may permit you to end the lease and obtain another vehicle at their discretion. They may charge upto £250 for this, again at their discretion.