In a seriously competitive housing market, you're going to need a lot of money to compete with existing companies (as others have said). If you have a lot of money, why would you want to limit yourself to just houses? You may as well try to buy a better domain with more scope (assuming they can be bought for the right price).
Sunglasses.co.uk is a bit different and I definitely wouldn't compare it to Fashion.co.uk in the same way that Houses.co.uk/Homes.co.uk can be compared. A closer comparison would be Glasses.co.uk, but even then that's a completely different market too as the majority of sunglasses have no prescription whereas the opposite is true for normal glasses.
Back to Houses.co.uk, if you're not going to develop it and monetise then you need to find someone willing to part with £32,000+ in the future, which of course is what the risk/reward ratio comes down to. Sunglasses could be developed and monetised into a solid asset with a far lower budget.