Aviation is a price-based business. People are attracted by price and retailers know that to get your attention they have to offer low prices.
I suspect that you can find a trip for £548 but obviously not for the dates you've selected. Somewhere or other there has got to be some small print that says something along the lines of "fares may not be available on all flights and seats are limited."
Trading Standards may be able to encourage the retailer to improve their language if it's not legal/clear - but they will not force them to sell you a ticket for £548.
If you are unhappy then go elsewhere.