The simplest way to determine whether use of an offshore account is lawful is to investigate whether the owner of said account is utilising it for monetary gain and neglecting to properly declare income to HMRC, giving them a financial advantage over those who are paying tax correctly, and inhibiting economic cooperation and development. This behaviour amounts to tax evasion.
If you are currently taking advantage of the system in this way, you are liable to be investigated by HMRC and may be found guilty of non-compliance, resulting in a hefty penalty.