Edam- are you smoking? The City "caused" the decline of Britain's heavy industry? Maybe you should consult a history book.
The City of London did not become prominent as a financial centre to rival New York until the mid 1980's.
Britain's heavy industry was screwed way before then. By 1983, it was cheaper to mine and ship a tonne of coal from Australia to the Uk than to mine it in Yorkshire. This is largely due to the obstruction of the unions who refused to de-man and mechanise when they had the chance (i.e. to save some jobs by sacrificing some). Similarly, British Steel and British Leyland put the nails in their own coffins by refusing to wake up and smell the global competition, instead, relying on protectionist measures to keep them and their outdated work practices afloat. If a Japanese dude can make a family saloon better and cheaper than you, the city's got nothing to do with it. You're uncompetitive and you're finished.
We were never going to compete in large scale, low value add industry forever- that's the lot of the industrialising nation. Our crime is that we didnt adapt to changes in global competition- instead the Unions stuck their heads in the sand and hoped the government would stick on some import taxes to save them.
Btw, I don't agree that the city should carry on as before. I totally agree with you that we need a more diversified economy. It's a big problem. The City has acted as a massive brain drain for years- why become an engineer/ doctor/ scientist when you can earn three times as much working for an IB? I just think that when you're in the middle of a desert, you don't shoot your only horse before you find another one.