why should someone apologize for what some other people are/were doing - I don't see a point
There are numerous threads on PF with the common theme of "collective guilt and responsibility" as in what they falsely claim Poles owe to others such as Jews, economic migrants clamoring to enter the EU, etc.
And of course the biggest advocates of "collective guilt and responsibility" (i.e., Harry, Delphia, Doug, the gay Turkish whirling dervish, etc.) also engage in mental gymnastics whereby when this collective guilt tripping comes around to applying to them then all of a sudden they want everyone to see them as atomized individuals with absolutely nothing to do whatsoever with the jingoism and xenophobia of their kith and kin back in Blighty or their literal genetic cousins over in America.
In the case of this thread the British do need to take ownership of what is going on back in their homeland and exert peer pressure on their compatriots. They can do this by announcing how they have written to their MPs to have this offense Polonophobic sign torn down. They can announce how they have donated to the crowdfunding effort to help the Polish father in Britain sue to have the offense Polonophobic sign removed.
At the very least they need to stand up and say they are ashamed to be British for what their countrymen are doing to Poles and acknowledge that it is not an aberration but a Stage 4 social cancer which needs to be confronted and eradicated.