The excellent book "The Hop Industry" by Hubert Parker contains some fascinating information on historical hop use. The use of American hops in British beer started long before people wanted beer that tastes of grapefruit.
The importation of foreign hops on a large scale seems to have started in 1862 when customs duty was removed from hops, though English growers were able to hold their own as excise duty was also removed and the market was expanding:
"... in these early years of large importation in the mid-century, Belgium, the Hanse towns and the USA were the places of origin. Later in the century the largest supplier of foreign hops was the USA, with Belgium second and German third. In 1900, out of an average annual import of about 200,000 cwt., about 150,000 cwt. appears to have been the average consignment from the USA, while Germany and Belgium would probably constitute as a rule about 12,000 cwt. and 35,000 cwt. respectively. The continuance of importations of this magnitude of hops which, though generally lower in price than English hops, were not, except perhaps as to a small proportion, cheap hops, indicates clearly the change that was entering into English brewing practice. Up to, say, 1860 English brewers were content with English hops. Thereafter, a mixture of foreign hops has been demanded. For supplies of these foreign hops, the largest quota came from America, whose hops in earlier years, it is curious to note, were esteemed indifferent hops of unpleasant aroma."
It's noted that American hops had high preservative value, i.e. alpha acid content, which is why despite reservations about their flavour English brewers were keen to use a proportion of American hops.