We have a community transport system in my town and I know there are similar schemes in many other places. The schemes use volunteer drivers who use their own cars, there is a set charge that is a lot lower than a taxi.
For example a return trip to my nearest hospital ( 13 miles away, so 26 miles total ) is £15 ,of which you can claim £11 back as travel expenses at the hospital if you are on a low wage or benefits. A couple in their 80s would definitely qualify for the travel expenses refund.
If they qualified for a mobility car but can not now drive they should claim the cash alternative ,around £230, every 4 weeks. This cash is to enable disabled/elderly people to get around by using Taxis etc.
It is worth getting the CAB to do a benefit check for them, with your Dad and Mums conditions there are a lot of benefits they should be getting. Severe disability benefit , carers allowance,attendance allowance for example. If fact they probably are both entitled to those benefits which means they should be getting double the single persons allowance.
The government will not just pay these benefits you have to claim them and the CAB are experts at making the claims.For example my wife gets disability living allowance, we tried 3 times to claim it ourselves and were turned down each time. We got the CAB to claim on our behalf and got the full allowance paid in just 5 days and had it backdated 6 months !