I have found the following link.
https://www.gov.uk/guidance/ukvis-new-f ... ed-to-know
It's a first time I have heard about this - can I travel safely before Brexit or do I have to await the decision?Waiting for a decision on your application
If you are applying for leave to remain, or indefinite leave to remain, you should not leave the UK until you have received a decision on your application. If you travel outside the “Common Travel Area” (CTA) before the decision is made on your application, your application will be treated as withdrawn.
If the decision is made while you are outside the UK and your current leave has expired you will not have a right to return to the UK and you may be refused entry at the UK border.
Also, when I apply for citizenship would I be able to go to Poland?