Firstly, you wasted 500 quid because the insurance was completely unnecessary. Your statutory rights mean that the retailer is obliged to repair or replace any item that fails 'due to an inherent fault' (such as poor quality components or poor manufacturing standards) for up to 6 years after purchase. (Within the first 6 months, there's normally an automatic assumption that any problem must be due to an inherent fault. Thereafter it's up to the purchaser to show that, based upon the balance of probabilities, an inherent fault is the most likely cause of the failure. In practice, retailers are usually totally unable to suggest any other reason why the set might have failed, so there's no need to worry about proving 'inherent faults').
If Comet is simply remedying the fault under their obligations which come about through your statutory rights, they're simply obliged to provide the same model as you originally purchased or, if that's no longer possible, a model which matches the original specification. (It wouldn't matter if such sets were now selling for �1 each. You'd still only be entitled to a direct replacement, rather than a set to the value of �2000)
It's possible (but probably extremely unlikely) that the totally pointless insurance you've purchased might impose an additional contractual obligation upon Comet. (The only way to be certain would be to read the small print). Unless it does, you'll have to accept the same model which you originally purchased, or the nearest direct equivalent, irrespective of the current retail price of the set that you're offered.
Chris