Before concluding that you have problem with BCU and take actions on BCU I would first do the following:
- take out the relay from fusebox and make connectivity test from switch positive to corresponding relay terminal in order to be sure you are testing correct relay.
- test that incoming terminal on the correct relay have 12 V power applied
- if above test if succesful then I would bridge two corresponding relay terminals in the fusebox and by that confirming BCU ground signal to relay coil is realy in question here
If I am certain I have ground connection problem with BCU I woul definetly contact Paul from
https://www.technozen-electronics.co.uk/
for all questions related to BCU.