Provided the money is in a UK-regulated bank or building society account, it's protected under the Financial Services Compensation Scheme, meaning...
Up to £85,000 per person, per financial institution is guaranteed.
For large savings, to keep it 100% safe, simply spread it in a number of accounts, not putting more than £85,000 in each. In fact consider spreading money even if you've under this amount, as if you needed to claim compensation it would take a while to get your cash back.
The exact rules about what counts as 'UK regulated', the links between institutions, and holding joint accounts make it more complex. For full info see the detailed Are Your Savings Safe? guide.