The German Reinheitsgebot ("beer purity lawe") from the 1500's (1516?) meant that it was only permissible to use four ingredients in the production of beer - water, malt, hops and yeast. Nothing else. This still applies to this day.
Here in the UK however, they can use added sugars to increase fermentation (such as sucrose, or inverted sucrose etc., rather than the natural maltose from the malted grain), artificial sweeteners for final taste, colourants and head-retaining agents for presentation, sulphur dioxide / sulphites, preservatives to prevent secondary fermentation and to increase shelf life etc. etc.
So when your foreign-sounding lager says in the small print "Brewed Under license in the UK", it is so they don't have to adhere to these laws.
Becks is imported from Germany, rather than brewed here under license, so the point of the advert is to highlight the brands four, simple ingredients according to the Reinheitsgebot.