You have three possibilities.
In order likelihood...
Most likely on current trends is the right hand switchgear.
Less likely but a possibility is the battery.
Least likely, because we haven't seen many, if any of these failures lately, is the immobiliser ring antenna.
The switchgear and the battery are a swap out job.
If you suspect the ring antenna then you can isolate the fault by using a second antenna module with the original still installed in the top yoke without having to strip it all apart.
I carry a spare ring antenna for this very purpose. Works with most BM's as long as you have the 'spare' ignition key with you as well as the original.