Regarding NCD, as far as I understand, you will lose the 10 years you had prior to joining Motability. NCD must have been earned within the last 2 years.
However, some (most?) insurers will accept the RSA letter to give an equivalent NCB from the years that you were on the scheme.
Having said that, I’ve got 12 years NCB on one private car, and had to insure another (Focus 155 MHEV) with 0 NCB in a rush last year, no time to shop around etc. The price difference wasn’t much at all, despite still just about being in my 20’s and unable to work due to disability. I do live in a fairly nice area and don’t have any points on the licence, so that may have made a difference.
Think I’ve mentioned in a previous thread of yours, I’m not a fan of the 155 MHEV. It is much better than the original 1.0 EcoBoost in terms of power and torque, but it’s not very smooth or refined compared to other modern engines.