of Goods Act says that the item has to be of "merchantable quality". It does allow for return of items that are "materially not as described", which in your case is the wing which you asked about and were told was fine (you can get into a whole minefield here but it won't come to that).
The Act does allow for items to be sold that are "damaged" if the damage was so obvious that you should have seen it at the point of purchase. You would indicate that you did, but that the salesman told you it was fine, it was only with further inspection that the extent of the damage became apparent.
Their choice now is repair, refund or replace.
If you accept a repair you can't later ask for a refund, it's one or the other, not both.
With a refund they can cancel the finance and take back the car, returning yours or they can reduce the price of the car and notify the finance company (don't just let them give you cash, you'd have lost your rights to repair and still have to pay finance on the amount they give you)
If you go for a replacement make sure the finance is all changed correctly so you're paying on the correct vehicle.
If you go for a repair you want a full replacement repair, not a filled and knocked out job, this is the ONLY repair an insurance company would accept, anything less devalues the car, I'd also ask them for a copy of the HPi report to make sure that the damage wasn't a claim (unlikely but possible). Whilst being repaired you also need a car, not your old one, you're now paying for a higher value vehicle, you're not going to pay that to drive around in your old one.
It's likely to be fine, big company, mistakes happen, they'll sort it.