The academic literature of the last two decades suggests that a Brexit would be rather at odds with the British own economic interests insofar as it would depress productivity gains in the industrial sector and thus weigh on growth and employment.
PDF - 127 K
The best of the recent academic literature suggests 3 ideas on this issue:
a) the depressing influence of a Brexit on the British industry would not so much transit through its direct effect on trade and foreign direct investments than through;
b) an indirect, significant depressing effect on industrial productivity which;
c) could lessen the GDP growth rate by around -0.5% per year until 2030.
This order of magnitude is a lower bound for the total impact of a Brexit insofar as it only refers to the effects transiting through the industry and not, for instance, through the UK financial sector. Additionally, most of the detrimental effect of a Brexit would be concentrated on SMEs, because the SMEs import and export more, on average, than the average firm.