To be honest, I'm more concerned about the UK side of it all, since it involves filing tax returns and registering as self-employed (while still being normally employed). I'm going to make an appointment with my local tax office as soon as the ITIN arrives.
Actually it's dead simple. If you earn under £300 (IIRC) a year from overseas you don't need to declare. If you earn over that, you can declare online, after deducting expenses etc. Takes hardly any time. Registering as self-employed is also a doddle.
But yeah, talking to the Tax Office is a good idea so you can see where you stand.