(1) The British learn history differently from that embraced by the Vatican and the church. They admire Elizabeth I, recall the defeat of the Spanish Armada sent to destroy Protestant England, recall the conspiracy to demolish the Parliament building, etc. Many of them also don't regard Henry VIII as the founder of British Protestantism. They see him as a Catholic who, for personal and political reasons, became schismatic, but he never adopted Protestant theology apart from rebellion against the Papacy.
(2) The sex scandals have been used very successfully to undermine Catholic influence. Much of the bitterness is directed against the hierarchy which, many feel, protected the predators so as to protect the church.
(3) Secularism, of course, has become rampant. Among Protestants there has been an upsurge in evangelicalism. Evangelicals are less likely to relate well to Catholicism than Anglicans. They share certain social policies with Catholicism (e. g., against abortion), but they take sharp issue on theological matters. Many secularists and evangelicals regard Catholicism as a relic of medievalism with its various 'superstitions' and such.
(4) It always is alienating to some women to witness an all-male hierarchy on parade.
(5) The Pope, in carefully chosen words, insists that Catholicism is the one and only true faith, that ecumenism means 'returning' to the Mother Church. This alienates many who are not anti-Catholic, who crave better relations, but who are offended by claims that Catholicism alone teaches the full truth. Benedict XVI had stated that Protestant churches are properly only 'ecclesial entities' and that Protestant clergy cannot validly administer communion.
(6) Some sophisticated critics of Catholicism feel that the Vatican has exploited Cardinal Newman, that Newman was in fact a conciliatory moderate who would have taken issue with the conservative trend within the Vatican today.
There are others, but perhaps this is a start. I could be wrong, of course, but that's how I interpret Britain's anti-Catholicism so far. Actually, I was surprised to see so little of it. The welcome given the Pope was quite warm considering that Catholics are perhaps less than 10% of the population, and a high percentage of those are of Irish background or from overseas areas once part of the empire.