DPA Principle 7
states that you as a data controller are required to protect personal information from accidental or unlawful destruction. I humbly suggest that the destruction of the landing cards of the 'Windrush' generation was in breach of principle 7 of the DPA, and therefore unlawful.
(Apologies if I m repeating an earlier post, but I've not actually managed to read them all yet.)
I should probably admit that one of my parents was a refuge from NAZI Germany, so I'm possibly a bit biased when it comes to protecting immigrants' right.