Yep that looks like the one (we have a Royal), its a standard part for many makes of mover. Not seen them in Halfords.
If your switch is in the battery compartment, tie a thin piece of nylon cord to the key and tie the other end to one of the loops used for the battery retaining strap. You'll never lose another one then.
Keep isolator key on your van keyring then you wont lose it or forget to isolate your mover as not all movers switch off after use, some mover control boxes just sit and wait for command from remote control and you will flatten your battery in no time at all