Is the term “ring-fenced” the British equivalent of “lock box’?
I would assume so. The context of the phrase suggests that the term denotes an 'untouchable' revenue stream, but we've all seen so very many similar plans that are corrupted and the monies end up going into the 'general fund' or some such hocus-pocus.
One thing that Socialist accountants and bureaucrats do extremely well is economic sleight-of-hand.