I think Unis tend to take native language A-levels with a pinch of salt. This isn't to say that they aren't perfectly valid A-levels and may be able to get you out of a tight spot on results day, but this will vary massively between unis.
Frankly your best bet is likely to ask your head of 6th form/uni applications person, as well as the head of the languages department at your school, they will be able to give you the best advice specific to your situation.
Its important to remember that A levels are tricky, they are not like GCSE languages where you just need to be able to speak/understand a bit of the language, there are cultural and research elements as well. You will need to learn hundreds of (useless) facts for your speaking exam, as well as pick a topic and create an IRP (individual research project). There is also the writing paper in which you will be asked to learn and write an essay on one film and one book. Paper 1 (the listening and reading paper) is likely the only one you could swan into.
You therefore need to consider how secure you are in your other subjects before starting Polish as unis are all happy with 3 good grades (mostly As). At my school we are not allowed to take 4 A levels (except further maths) because of this.
Ask yourself if you can cope with the extra workload, and if this will help you in your goals.
All this being said, languages A levels are really enjoyable and immensely rewarding.