Comparing John Lewis with the Competition

Mr_Singleton

When you buy an own brand item from John Lewis how do you find an equivalent item to compare it with?

So for instance looking to buy a Kingsized
John Lewis Crisp & Fresh Egyptian Cotton 400 Thread Count, White duvet cover

But what should I compare it with in terms of cost and quality?

I notice that The White Company do a 400 count cotton duvet but are £30 more expensive.

Which if any would be the better buy?