no, they can't fail it for airbag light on as it's not a testable item, unlike abs light for example. if they do fail it for airbag light, get them to show you in the mot handbook, and you can't take the bulb out as it's two small led's that would require taking the clocks apart and de-soldering.
so either leave it or put some tape over it. it was me that put tape over squarepants' light, when it was my car, and no probs at all.
with regards to the horn, get a seperate two pin push switch (doesn't even need to say horn on it (just make sure tester knows where it is before you leave it for mot)). run an earth to one side of the switch and connect the other pin to the horn wire under the steering column.
to get the horn button to work on the steering wheel, you'd need to connect the horn wire to the brass contact ring on the back of the boss. like on a pre-facelift. it's difficult, but i did mine, so can be done.
