It all depends on your status.
If you set up a limited company, for instance, you could leave all your earnings in the company (i.e. not take a salary) and only your profits will be taxed at 10%.
As a sole trader, you will pay taxes at 42% if that is your tax rate, however you can deduct your professional expenses incurred in your side business.
To avoid paying taxes, don't make a profit for the first year or couple of years, which in my opinion is the best way to get your side business off the ground. (Please do not confuse this with evading tax, embezzlement etc!)
The above is my experience and your circumstances will vary - please check with your accountant.