If the safety breaker used in the new isolator switch is of an RCD design, then the housings can be metal clad at both ends of the new sub-mains supply. There is no need to insulate using things made of plastic in between the new consumer unit and old installation. The OP (FreddieLives) should know what types of safety breaker devices to use on this type of sub-mains installation or he shouldn't be touching it at all, in my opinion.
A new earth spike at the out-house would protect both the new SWA supply and the garage CU without interfering with the existing house system. The only connections made inside the house would be the tails and Henley blocks from the meter to a new isolator switch with a properly rated safety breaker device in-built.
The earth fault loop impedance would not have to bring the new installation to the same potential as the house, as the new supply and garage installation would become a totally separate entity from everything around it. So there would be no need to install equipotential bonding or insulated anything between all parts of the old and new systems. The new earth spike at the garage would be between the sub-mains cable and the out-house consumer unit, and would mean a more secure method of fault detection for all of the new system. If done properly, then the SWA cable and new consumer unit and wiring system in the out-house would share the same earth fault protection between them.
Nothing over complicated in it, and all safe.