An awful lot of ex-Motability vehicles go via British Car Auctions various sites across the UK. They have ex-Motability sales a few times per week:
https://www.bca.co.uk/buy/useful-information/types-of-auction/motability/
To bid, you have to hold a ‘MY BCA Silver Card’ which is only obtainable after you have bought 3 vehicles from them and ‘upgraded’ from their blue card:
https://www.bca.co.uk/buy/Useful-information/MyBCA-Card/MyBCA-Card-Silver/