If you do not want to have this as fixed wiring then it would be far easier just to run an extension lead to the garage for when you want power, this would avoid any problems with poor connections.
If you are going to use the SWA then the only real answer is to have it as fixed wiring. If the SWA does not reach to the consumer unit then you should extend the circuit back to the consumer unit with an appropriately sized and protected cable. Use suitable circuit protection at the consumer unit and set up a satiable circuit in the garage, if needs be just one double socket.
However you do need to do your calculations for the circuit and ensure you have the appropriate protection.
If you are going to use the SWA then the only real answer is to have it as fixed wiring. If the SWA does not reach to the consumer unit then you should extend the circuit back to the consumer unit with an appropriately sized and protected cable. Use suitable circuit protection at the consumer unit and set up a satiable circuit in the garage, if needs be just one double socket.
However you do need to do your calculations for the circuit and ensure you have the appropriate protection.