I think one needs to make a distinction between 'British Opera' as in "the performance of opera in Britain" and "operas composed by Britons", and they are very different. The latter are terribly, terribly neglected, even among people who are interested in other British music.
Indeed, Britain has produced many fine composers whose operas could grace the stage and provide a vibrant national repertoire - Balfe, Benedict, Bliss, Boughton, Brian, Britten, Alan Bush, Delius, Hoddinott, Holbrooke, Holst, Lloyd, Loder, MacCunn, Macfarren, Mackenzie, Scott, Smyth, Stanford, Sullivan, Goring Thomas, Tippett, Vaughan Williams, Wallace, Walton ...
Unfortunately opera is a high-risk extravagantly-expensive business - in a crowded market they (apart from Britten and about half-a-dozen of Sullivan's lighter works) are clearly not perceived to be commercial propositions. Lacking the glamour of a foreign product, British opera (when it has been considered at all) has always been regarded as parochial, something for interested amateurs at best - it is, unfortunately, very difficult to envisage this situation changing to any great degree.