I think it would be difficult to build up British industry again, after it has been run down so badly

Most heavy industry (mining, etc) has been closed: the mine shafts and quarries are flooded, and would take many years to bring back to working order. North sea oil is running low, and we seem to be buying in Russian gas too!
Other industries (steelmaking, some power production) are owned by companies based in other countries (such as Corus and EDF) - so a percentage of the revenue (and doubtless the top jobs) go outside Britain. Light industries and manufacturing (e.g. the car industry, aerospace, household goods, electronics) is also largely owned by non-British companies. LDV is the last notable British car manufacturer and they are now in real trouble...
So Britain must make a VERY LARGE investment in industry if we are to exist alone: either excavating our resources, or buying back our manufacturing bases. And we would need to educate a new generation of bussinessmen and women who are interested in making a company work long-term, instead of just getting rich quick. I have no idea where that kind of money and culture change might come from - we would probably need to borrow it from abroad
