The most important point is what happens if the 0845 number is called.

If it still works and connects callers to the callcentre, they stand in clear breach of FCA regulations in force since October 2015.
If the number plays a message telling all callers to hang up and re-dial a new number, there's no real problem other than a bit of inconvenience to those callers.
If the number returns Number Unobtainable tone, they make themselves look stupid and disorganised. They also risk callers becoming scam victims if they start searching Google for a phone number as there are many fake premium rate phone numbers posted on scam websites to trap the unwary.

It is poor form that more than four years after the rule change they have still not managed to eradicate all mentions of their old phone numbers from their own paperwork, website, and apps. It is prudent to realise they will never be able to remove them from third-party websites. Those pages may well promote old numbers for many decades (just look at how many web pages are still out there with old London 01, 071, 081, 0171 and 0181 phone numbers detailed on them).