A few points from dealers POV
Any customer that ever has to claim on GAP ins is a customer for life because its such a bonus when needed.
Gap ins (Retunr to invoice) cost a dealer about £150-£200 and depending on how hard you haggle i think between £250-350 is reasonable
Dont bother with the pay off finance as its not gonna make much of a difference unless you are buying a high depreciating car with low deposit over the never never. In which case i would say you are buying the wrong car.
Return to invoice covers any SHORTFALL between the insurance settlement and the invoice price, usually upto £10000 limit which is more often than not enough. In 3 yrs your car would have to depreciate from 16k to 6k to have any shortfall for the cover.
Is it worth it? Well thats for you to answer but i would say you will be in a shortfall situation quite quickly with your car and at 24-35mths old it would be extremely worthwhile in the event of a total loss.
Beware though, some car policies give new for old in first 12mths, so you are really only paying for 24mths cover.