Tax system in Great Britain works a little bit different than in Poland. Tax year starts and ends at 5th of April. To reclaim tax refund you need to file appropriate documents in territorially competent unit of Inland Revenue (British tax authority).

It is possible to receive tax refund up to past 5 years. In order to settle your tax you will need to provide P45 or P60 document received from your British employer. If you don't have any of these, you will need to contact your employer to issue them to you or their replacement in form of so called Statement of Earnings.

Tax refund procedure usually takes up to 6 months. Unfortunately, waiting period can be longer, in extreme cases even above 1 year but this usually happens in problematic situations (like lost documents, lack of needed documents and others). 
If your earnings in Great Britain did not exceed tax free amount, then it is possible to reclaim even 100% of advances on your income tax. Tax free amount is determined by Inland Revenue and changes every year, currently for year 2010-2011 this amount equals 6 475 GBP.

People self-employed in Great Britain are obliged to file full tax return until 31th October (in paper form) or until 31th January (in electronic form). If you fail to do that, British tax authority may impose you with a fine up to 100 GBP.