"Fewer UK-born Bangladeshi women are returning to Bangladesh to find husbands to bring back, robbing the industry of another labour source."
Leaving aside the gender and racial discrimination in hiring restaurant staff, the reason why this 'labour source' has been cut off is a deliberate act of government policy. Concerns about forced or coercively-arranged marriages have been growing, as concerns about immigration in general. Hence the introduction of the spousal visa: if a non-EU national marries a UK national and wants to immigrate, they have to prove that their spouse earns over £18k, which is slightly above an average year's pay on minimum wage. If a family is traditional enough as to prevent their daughter from working outside the home, that also prevents them from importing a spouse.