Presley had proven that there was a market for rock'n'roll in the USA. Countless imitators proved that a similar market also existed in Europe, but they had to twist and reshape the sound and the lyrics. Britain had a tradition of importing forgotten bluesmen from the USA, and became the center of the European recording industry. European rock'n'roll was less interested in innuendos, more interested in dancing, and obliged to merge with the strong pop tradition. European rock'n'roll, from Italy to England, was much more melodic than the original. In fact, none of the most famous British, French and Italian rockers qualify as a "true" rocker: they were still pop singers. Johnny Kidd and the Pirates was the notable exception, particularly with Shakin' All Over (1960).
A clear difference between USA rock'n'roll and British rock'n'roll was that USA rock'n'roll had a relatively rural (Mid-western and Southern) origin, whereas British rock'n'roll was strongly urban and industrial from the very beginning. London, in particular, was experiencing a rebirth. Just like all other European capitals, the "swinging London" was awash in money and enthusiasm. But, unlike most European capitals, London (and all the other British industrial cities) had a vast reservoir of poor, alienated youth, the price that Britain paid for being the most industrialized country in Europe.
A more crucial difference was one that truly changed the way rock'n'roll was perceived by the public: the British had a stronger concept of the "group" as opposed to the "individual". The rocker had been a typical USA phenomenon: a musical transposition of the "loner", the misfit, the nomad that is frequently celebrated by USA novels and films. Britain had a different musical tradition, that was grounded on the orchestra: to them, a jazz or blues or rock "band" was simply a small-scale orchestra. Also, Britain had a different social background: the "gang" prevailed over the "hero". The names of the first rock bands were aping (often in a mocking way) the names of the gangs that operated in their territory. USA rock'n'roll had translated the sense of individual frustration into the cult of personality. British rock'n'roll translated the sense of collective frustration into the cult of the group. USA culture, particularly in the Midwest and in the South where rock'n'roll was born, emphasized identity, but British culture, particularly in the industrial cities, emphasized the loss of identity in favor of membership in a group.
When the London bands pared down the "trad" orchestra to a guitar-driven combo, British rhythm'n'blues was born (namely Rolling Stones and Yardbirds). When the Liverpool bands replaced the instruments of skiffle with drums, bass and electric guitar, skiffle mutated into a new genre, that was renamed "Mersey sound" (or "Merseybeat") in 1963 (Mersey being Liverpool's river). The Casanovas (later Big Three) were perhaps the first of the Merseybeat groups (or at least the first to play at the Cavern, in may 1960).
Just like Presley's success spawned a generation of rockers, the success of the Beatles spawned a generation of rock bands. As the Beatles were easily accepted by the mainstream (they conformed with the conventions of the white, conservative "teen idol" era), rock bands that imitated them were also tolerated.
True to their musical roots, the Beach Boys and the Beatles continued to produce melodic music, and became more and more sophisticated in their arrangements.
The Beach Boys (2) had the idea first. Ballads such as Don't Worry Baby (1964) were a synthesis of Phil Spector's "wall of sound", Chuck Berry's teenage vignettes, doo-wop's stately four-part harmonies, as well as the cornerstone of a new form of pop music. That form was born with I Get Around (1964), the greatest of their car songs, Help Me Rhonda (1965), the most acrobatic of their multi-part vocal inventions, Barbara Ann (1965), the most anarchic of their geometric constructions, and Good Vibrations (1966), the first pop hit to employ electronic sounds. Brian Wilson, the genius behind the Beach Boys' sound, became the quintessential eccentric of melody, particularly on Pet Sounds (jan 1965/mar 1966 - may 1966) and the "lost album" Smile (aug/dec 1966, but released only sep 2004). Brian Wilson created a unique role for himself when he quit playing: basically, he was a composer who had a band to perform his repertory. The Beach Boys became immaterial.