From a technical perspective once the Americans were in possession of German swept-wing data, as well as working examples of German and British jet engines, there was no looking back. The sheer scale of the economic and technical resources of the big American companies (backed by Government orders/contracts) was far greater than what any British company could expect. Boeing in particular benefited from military orders for the majestic swept-wing B-47 jet bomber that flew a full 18 months before the Comet, and by then it had already commenced design of the eight-engine B-52 jet bomber. The knowledge learned from these two projects alone would have been invaluable to the design and development of the 707. (Famed Boeing test pilot ‘Tex’ Johnston flew all three types, not to mention travelling in the cockpit of an early Russian attempt at a civilian jet airliner (Tu-104) that he described as rubbish compared to the 707).
Comparing the subsequent 707 with the Comet, the former was bigger, more powerful, faster, and could fly greater distances and at higher altitudes. Of course Comet struggled to regain its shattered reputation after the series of disastrous crashes, and although the Comet 4 was vastly superior to all the previous Comets, it was too little, too late.
As Feather mentioned, the marketing of the American jets was also far superior to the British marketing of its own aircraft, and of course, America’s own domestic market was significant in itself. Boeing used Tex Johnston himself with great success to showcase the 707 to the world