Interestingly, I think you can argue that the brits were at or near the top of the automotive world in the postwar on into the 50s, the jag 120 offered world beating performance at a much lower cost than comparable cars coming from Italy, and later Germany (300SL). The Triumph TR2 would outdrag any stock American car to 60 MPH when introduced, at the end of their cycle on top you had the advanced E-type (DOHC, four wheel discs, four wheel independent suspension) and the Mini, which became the template for a good chunk of the world automotive production 25 years later or so, by which time most all mass production lower to middle class sedans were unibody traverse engine front wheel drive for the packaging and production cost advantages. But then, with the exception of low volume manufacturers like Lotus, development stagnated, and the cars being sold in the late 60s and early 70s were basically the same or evolutionary continuations of late 50s design (see MGB, Big Healey, AH Sprite). American cars offered a hard to beat combination of power, reliability, comfort, space, and a higher level of technical sophistication than they are usually given credit for the in 60s (for example, best slushboxes and AC systems in the world, great bang for the buck v-8s), but this didn't really kill the British car in America. The rise of the European and Japanese cars in the late 60s and early seventies, which dealt with increased concern about fuel and insurance costs, and probably most importantly also dealt much better with US emissions regulations than the British or the American manufacturers, coupled with the fact that the European and Japanese cars were more direct competitors with the British cars, and the brit cars were getting to be really old designs is what sounded the death knell for the British sports car. MGs and Healeys were based on British sedans, if you look at the production numbers for many of these cars they often weren't greater than the numbers for the sports cars, there weren't the economies of scale needed to make a profit and put money back into development, and what money there was for development went into meeting US regulations instead of increasing performance or other marketable characteristics like body style. As production and design techniques changed, for example from frame on body to unibody, the cost of tooling up for new models got more expensive as well. I remember reading one about one BL executive from the period who said the production volumes were just never quite there to make a profit, with no profit for development the problems just compounded themselves, the last great hurrah was the TR7, which sold in very good numbers to start. But the new wore off and the reliability reputation hurt it as well. In fact the later convertibles built in 79-80 were pretty good cars, my brother bought one new and drove it for years, but even the glowing reviews of the TR8 could not save them at that point and the imports besides Jag went away, followed by most of the domestic industry in the next 15 years or so.