With regards to fans, you could get a very cheap mains desk fan (especially a tower-type one should be fairly easy to fit). Then wire that to the switch you use to turn the compressor on. I.e I'm guessing you already have a cable to the compressor (or it wouldn't run). From this cable, somewhere, you can hook the fan up. Then you switch this cable on/off when you use the compressor. In UK you typically have switched sockets, so it would all be switched from there.
Another thing that struck me is whether you should have the fan on the intake or exhaust side. If you have a big enough fan that can create a little (just micromicromicro) pressure inside the box, then putting it on the intake side would perhaps aid the silencing as the pressure would force the air out in a constant stream. That's how the silencers in the cars work, they create a bit of pressure to even out the "bangs".
Either way, I think you're onto a winner here.
I know most people do this, but your not supposed to switch things like compressors on with the switches in plugs, your supposed to use the breaker they have on them.
Most UK plugs are at the very top of there rating with a 3hp compressor, turning it on and off at the plug could cause you some problems with the switch in time.