Boeing Hatfield visit
That visit was years later and to a different manufacturere - BAC rather than Hawker Siddeley controlled Hatfield.
There are similarities however as Boeing had fostered a relationship with de Havilland in the wake of the Comet fuselage problems and de Havilland had correctly shared its knowledge gained through the disasters with the wider aviation world.
Boeing had offered the B720 to airlines in the US but most indicated that they wanted something smaller, with good short field performance but with range. Douglas had signed an agreement with Sud Aviation to market the Caravelle in the US and it may well have been that Boeing were looking for a similar deal on the Trident as they perceived the aircraft to be well ahead of their project (it was) and were concerned that it would breach the US market because, at the time, Eastern, American and TWA were all interested in a 3 engine layout and a high performance wing.
Boeing had done a great deal of work on various wing possibilities and expressed interest in the Trident wing. They were shown the two distinctly different Trident designs and, rightly, decided that the T tail, three engined layout with Medway engines and originally specified wing was the way to follow.
Having either failed to reach a marketing agreement re the Trident (or decided not to in the light of what they had been shown) they went away and continued discussions with the US carriers and in June 1959 commenced work on the design that was announced in December 1960 as the 727.
Meanwhile the much bastardised and hobbled committee/BEA produced Trident 1 first flew on January 9 1962 and failed to attract any US interest. Boeing flew the 727 on February 9 1963 having caught up, partly due to being able to use a number of common parts from the 707/720 and a great deal of expertise in assembling large airliners quickly gained from those and the KC135 programmes.