I don’t mean to be an absolutists and say it could never under any circumstances leak if handled poorly, but even if you just dumped it in a hole in the ground the odds are against that.
We primarily don’t have long term storage for us becuase it’s too valuable and we’re only currently using a small percentage of its potential energy. In other words we are not done with it.
Long term storage that exists like Yucca Mountain are heavily engineered to remove risk.
But if we’re talking about great brittian I mean, build storage on the French model, or pay France or the US for storage, or just do what you always do and toss the damn things into the Thames.