Is the increase, from £250 that the government deposits in banks to £1760 when the son reaches 18 years of age, considered usury (riba) and unlawful (haram)?

The increase on the original amount paid by the bank to the child upon reaching 18 years of age is usury (riba), because banks in those countries deal with usurious loans and borrowing, not with Islamic mudarabah (profit-sharing). The fixed increase on the paid amount confirms this. It is obligatory not to put money in the bank and to invest it in permissible ways (halal). If the state mandates placing it in the bank, then it should be in a current account. If it mandates a savings account, then the increase on the principal amount is not owned by the child and should be spent on the general welfare of Muslims, such as the poor and the needy. If the child is poor and needy upon reaching maturity, there is no harm in him taking all or part of it.