@Jason23 Assuming that I understand your figures correctly;
You are saying that the AP of your chosen car, including the full cost of optional extras, will be circa £8k? As it’s a Motability car, you have to add the bigger cost, which is the benefits that you sacrifice, which adds another £13k minimum, so a total cost of £21k, or £7k pa.
The personal lease example that you give seems to be a 4 year lease (12+35) which adds up to £19.5k plus £1.1k for options = £20.6k, or £5.1k pa. The lease car (being new) would obviously include a warranty and breakdown cover, and excess mileage charges are less than Motability now charge, but you’d need to add the cost of insurance and the cost of adaptations which, unlike a Motability car, won’t be subsidised (but will be VAT free).
In truth, if it were me, I wouldn’t take either option! I wouldn’t be prepared to pay over £20k to lease a car that I will hand back after 3 or 4 years with nothing to show for it when, for less money, I could buy a very nice second hand car which, having already taken the initial depreciation hit, would cost significantly less to run and leave me with a sizeable deposit (trade in) next time. The £330 (and rising each year) extra benefit income every 4 weeks will cover unexpected costs (tyres etc) many times over.
However, if this isn’t something that you can (or would) consider, and you are set on a new car and prepared to pay the premium for one, a lot of it will depend on how much mileage you do (higher mileage customers are being driven away from Motability, as their excess mileage charges are punitive), how much the adaptations cost and what your risk appetite is.
If, once you do the maths for your personal circumstances, there isn’t a large price difference, my view would be that if Motability offer a car that you want, they will probably remain the safer bet. As @shaan200 correctly points out, they are very good at allowing early terminations if your circumstances change, which a private leasing company won’t. If the cost saving is significant then jump ship, but make sure that you look into everything thoroughly first, including the cost of insurance (high for EVs), tyre costs (heavier cars will generally consume tyres quicker) etc. My thread “We’ve left Motability – Tips for others considering leaving” has helped a number of people who are making, or considering, the transition.