You need to get permission from your CO which usually isn't a problem but depends on your particular CO's policy. It depends on what type of work you want to do, how often you intend to undertake this work and most importantly whether it will impact on your military service. Many people do it though which is handy to have a bit of extra cash now and then. As far as taxes go, you'll be taxed the same as usual on your service pay, but you will pay basic rate on your second income which is 20% at present. With national insurance your talking about taking home 75pence for every £1 you earn.