The explanation used to be that British manufacturing was unwilling to invest in plant and equipment so could not produce as much as other countries, even though some industries, like car manufacture were competitive, but manufacturing now only accounts for 25% of the economy.
I think it is because, politically, we are going along the US route of depending entirely on markets, with low taxes, low pay and, as a result, have low skills and, frequently, an unmotivated work force.
I am not routing for any particular political party, but compared with other EU countries our taxes are lower, our education is poorer and far more expensive to the person being educated (university fees). The employee is less protected when in work ( I do not mean HSE) and, therefore less valued.
In countries like Germany there are many more family owned small and medium sized businesses (SMEs) where the senior management are part of the business process and do not milk their businesses for huge salaries, which seems to be the norm in the UK.