I also have trouble reaching centre roof etc. At least I did with the Superbs and the (awful) Antara. Now with Mini it is dead easy. Still have to be dead careful though – getting lower back at wrong angle can flip another disc and leave me completely immobile.
Would normally use car wash at local Tesco but you have to leave it with them to do it. No sitting inside and waiting. So potential disease vector can smear stuff all over inside of cockpit. Thank you, no!
For heavy mud and wheels, I use a mains pressure washer. Otherwise I use stuff called “Eco-Smart RU” from company called Chemical Guys. It has been around for some 30 years or more and is a waterless wash and wax. Works brilliantly on everyday surface dirt. No need to wash. You spray it on then wipe down to remove surface dirt. Finish by buffing up with cleaner cloth. Works very well and is dead easy to use. Can clean the Mini in less than half an hour.