Opera in London is like that in no other city. With a b tradition of theatre (ie sensible plots and good acting) a parallel tradition has arisen carrying those over into Opera. The English National Opera performs all the repertoire in English - with translated libretti that surpass their originals for the most part. The Brits were doing this in 680AD even with the permission of the Church - the Bible was translated into stunning English verse at this time by Caedomon of Whitby. He also did versions of other libretti - a sort of dark-ages Jeremy Sams. Combine that with an insistance on good acting - never before did opera make such good sense. Our home grown opera composers - like Britten - placed theatricality at the heart of their works.
That said, if you prefer traditional opera with primadonna stars ("I don't like rehearsing," said one major international star, "I've done these operas so many times I don't need to") and high prices, then the recently refurbished Covent Garden Royal Opera House is for you. Tickets, despite a huge public subsidy are beyond the reach of most Londoners (about £80 for any reasonable seat, and that in 'the Gods') , - a few restricted view tickets are offered at reasonable prices a long long way from the stage - and most of the seats are reserved for corporate entertainment. With karoshi-suffering Japanese businessmen slumbering through the performance in the best seats there's hardly any impetus for good production, and top names are the draw here.
Covent Garden arose out of a battle between Handel's supporters and enemies - the previous 'Opera House' was Her Majesties' on Haymarket (where Phantom of the Opera now resides, like the degenerate progeny of the likes of Handel's 'Julius Casear' and 'Ottone'). The intense rivalry carries over and opera lovers polarise between the ENO and the ROH. That said the quality of the singing is usually better at Covent Garden, if the productions are staid.
The ENO has fought back with home-grown stars like Lesley Garrett and Rosemary Joshua - each season they mount at least one sublime production - and one dog, and the rest are merely good. They also have a good tradition of contemporary opera and with tickets from £6 - £40 (with the possibility of half price tickets in the Leicester Square booth) this writer's obvious bias is understandable. Visiting companies split between the two: eg: The Kirov went to the ROH, the Bolshoi to the ENO.