the problem is I have not declared any of the income I have earned thus far.
Have you paid tax on that money in the UK? If so, you have no problems. If you haven't, just declare it to the Polish tax office and pay what you owe.
this was initially a temporary move
Good, that means the 'centre of your vital interests' was not in Poland and thus you weren't a Polish tax resident (assuming that you haven't been here for more than 183 days, even if you have you'll probably be OK). Just contact your local tax office and tell them that as the centre of your vital interests was UK and not Poland, you were planning to pay the tax in UK and not Poland. However, now your plans have changed, you recognise that your tax status will have changed and you want to pay the tax in Poland rather than in the UK. They might be a bit surprised (a lot of Poles would just say nothing and pay no tax anywhere).
However, I find the Polish system so complicated and confusing that it makes things really difficult to do so.
I really isn't that bad once you get it set up. Yes it is annoying (for example you need to make three separate transfers every month to ZUS) but it's not that bad.
The main thing I am worried about is getting a heavy fine for not yet declaring some of the work I've already done prior to registering as a sole trader.
As long as you contact them and tell them that you think you owe them money and will pay it as soon as they tell you how much, you are very unlikely to have to pay more than the amount of the tax you owe and the interest which has built up since the date that they think you should have paid it. Don't worry about it. My former accountant screwed up my accounts for 2008 and then when they wrote to question the accounts she didn't bother to forward the letters from the tax office to me or tell the tax office that she was no longer my accountant. So they just helped themselves to my bank account: the full amount of the tax plus penalty interest and a small administration fee. No fine.