Some thoughts.
- Driving from London to Scotland wipes out a day either end of your trip. And it's not a relaxing or enjoyable drive. Renting in London for a tour of Scotland is not going to make for a relaxing holiday, so fly or train to Scotland (though you've left it late to get good deals on internal flights)
- Trying to tour Wales and Scotland in one week is unrealistic. All you'll see is the inside of the rental car. Prioritise what you really want to see.
- For the best rental deals just keep trying every option you can. Sometimes deals with airlines offer better rates - I can never understand the logic of the pricing structures of the rental companies