I converted your all caps subject so people wouldn't think you're shouting.
I just bought a 10 pack of Bosch relays (which I think are German built) here, but would imagine they would be relatively easy to find in the UK.
Finding a "Magnum Blaster" is probably a bit harder because it's an Italian horn with North American packaging. You might look at whoever supplies FIAMM.
I'd be happy to make up the whole harness for you too! It incorporates the Bosch relay right into it. Just connect and honk away. Shipping to the UK is no biggie. Plenty of my harnesses are in use there.