1) Andrews & Arnold Ltd don't have 4 million numbers. They have fewer than 100,000 geographic numbers, plus a few tens of thousands of non-geographic numbers, assigned to them by the UK telephony regulator. I suppose it's possible that they could have agreed to use more through another provider.
2) Trapping a few telemarketers and tormenting them for entertainment purposes is fine, as is making money for receiving these calls, but what will happen in practise is that they will answer a lot more "wrong numbers" from regular people who have mis-dialed. If they search their existing CDRs for rejected calls to their unused numbers they will almost certainly find that there are a few numbers that already receive many call attempts because the number actually dialed is similar to some other genuine number. Recording and using mistaken calls from "your mum" for entertainment purposes and charging her for the privilege is somewhat immoral in my opinion.
3) The correct behaviour is to reject unused numbers with an NU indication. Anything else is antisocial and profiteering, but they would be welcome to do this on their freephone numbers (where they are charged for the calls).
Note: I work for a telephone company that does have millions of numbers assigned, including many premium rate and pay-per-call numbers. We could make a significant amount of money from caller's mistakes, but that would not be right.