Malawi still free entry for Brits. Ditto Rwanda, SA, Namibia, Botswana. Most others in southern/eastern africa are $50USD. Can't recall what Ethiopian visa cost in London. Sudanese was c $50USD in Nairobi. Egypt $15 USD (and you can buy on the ferry going north).
By all means get them in advance but for those you can get at the border you won't really save any time. You stand in the same queue to buy the visa as you do to get the entry stamp on the visa. However, you will definitely save a day and some hassle getting Ethiopia/Sudan in advance (and you will need to sort Libya out anyway) but otherwise nothing much to gain IMO.
Importing the vehicle is usually the more time consuming part of crossing African borders in my experience. Some countries require a carnet eg Kenya/RSA, others don't recognise a carnet and insist on a TIP eg Ethiopia and some will either accept a carnet or issue a TIP eg Zamiba/Malawi.