Why the Royal Navy would avoid the Baltic

Could the Royal Navy keep Polish seaports open in a Baltic conflict?

Not directly; the answer argues for a northern carrier task group rather than committing scarce ships to the Baltic.

The answer treats the shallow, confined Baltic as an unfavorable theatre for the Royal Navy’s limited surface fleet and even its submarines. Those ships would contribute more by threatening the wider northern flank than by trying to keep Polish ports open directly. The alternative is a task group built around one or both Queen Elizabeth-class carriers, reinforced with British, American, and possibly other allied F-35s. Operating in the north would preserve sea room while pressuring the approaches between the Arctic and North Atlantic.