Virto asks how all these things work. The simple answer is lots of computers linked together. Airlines talk to car hire companies and hotels. Agents talk to airlines, hotels and car hire comanies. Deals get done (the precise nature varies). The seller, be it agency or airline or whoever then takes the prices they've negotiated, adds their profit margin and sells at an "advantageous" price.
A word of warning. UK price protection if you buy a "package" only applies if you buy from a UK company. I've seen websites with ".co.uk" addresses that turn out to be companies in Europe and beyond. Always remember WWW means World Wide Web and know who you are actually dealing with.
As with everything if it is too cheap it is likely to be unrealistic. A specialist dealer in model trains recently circulated a note to the effect that someone had copied his website with all the stock and pictures and was advertising the items normaly priced in hundreds (and even thousands) of dollars at prices like thirty dollars.