What is the ruling on the mandatory government pension scheme in Britain, and what is the ruling on the optional pension scheme in private companies, knowing that both involve investing the paid funds and disbursing them after retirement age?

Participating in a non-governmental pension scheme is a form of maysir (gambling/speculation) due to the possibility that the return may be less or more than what was saved, just as it would be forbidden if the money were invested in unlawful things. This applies if the participation is optional. However, if it is compulsory, there is no sin, but it is not permissible to take more than what was paid. The remainder may be left or distributed to charitable causes. The matter may differ in governmental pension schemes because the public treasury is responsible for expenditure.