When you say BMC do you mean Brake Master Cylinder?
The horn relay should be on the passenger side inner wing probably under a black plastic cover. Does the relay click when you press the horn button?
Here’s some stuff to check/test.
The relay should have 4 wires, 2 White/Green, Green/Red and Green/Orange.
You should have 12 volts at both White/Green wires. The Green/Red is the switched feed from the relay to the horns. Feeding this with 12 volts should sound the horns, if it doesn’t, suspect a break in the Green/Red wire.
The Green/Orange wire goes to the horn switch, you say that you have 12 volts at the horn switch so this wire should be intact.
Take the relay out and apply battery negative to the relay terminal that connects to the Green/Orange wire and battery positive to the terminal opposite (would have been the one of the White/Green wires). Observe if the relay click. If it doesn’t then its a defective relay. If it does click, then with it energised, check for continuity across the other 2 terminals, if there is no continuity then its a defective relay.