I happen to be the Technical Coordinator for the North American MGB Register, which serves all post MGA MGs, including the MGB, MGC, the post Abingdon cars, and the 1100 and 1300 sedans and of course the Midgets.
The British wiring codes are amazingly simple and easy to learn. Brown is power, black is ground. After it is switched in the ignition switch, Brown becomes white. When White is fused, it becomes the green wires, and when Brown is fused, it becomes the purple circuits. When Brown goes through the headlamp circuits it comes out as Red with Green stripes for parking lights, which do go through the fuse block and come out red to go to the lamps, and red with a white stripe to go to the dash lights., and bown comes out of the head lamp switch as blue for head lamp dip switch. blue with white stripes are main beam, and blue with red strips are dipped beams. Any wire that has a black stripe is a typically a switched circuit ground (like a lot of negative side two wire horn systems) and green wires tend to feed the convenience things on the car. The yellow wires are for the alternator or generator. There are more, but armed with this info you can troubleshoot almost any system on a British car.
Grounds are the big problem, keep them clean and assemble them with silicone dielectric grease. the little bullet connectors throughout the car need to be likewise cleaned and assembled with dielectric grease, and check to make sure the heat hasn’t overcome the springback of the metal for the secure connections. Some of the weaker contacts in the switching can greatly benefit from having a simple 12 volt relay placed in the circuit to take the electrical load and heat off the switches. Although early British cars typically have two fuses for the entire car, and the later ones have four (up to the early eighties) the electrical systems if properly cared for and kept in good order are almost bullet proof.