Earlier quoted context omitted.
My guess is that the ICO wont fine them very much as it did not include full credit card numbers. However they might up it for failings in process, lots of remedial measures etc. They might not even have PCI compliance issues alas. The management will argue that they knew nothing, although that is becoming less of a defence now.
Social engineering once you have the last four digits of the credit card number and the billing address is almost certainly enough to score full credit card numbers. (e.g. use them to reset password for e.g. Amazon account).
SSNs are worse, though. The last four digits plus your birth date & location might just give the whole thing away.