It's useful to be aware that NS&I (National Savings and Investments) has no limit on protection. It's basically a state-owned savings bank and therefore in theory as low-risk (with regard to default) as government issued bonds ("gilts" in the UK) which are generally considered the "minimum risk asset".
NS&I's website doesn't seem to go into any detail on this anywhere I could find other than a statement that
We're 100% secure
We're backed by HM Treasury. So all the money you invest with us is
100% secure. Always.
on their "Why save with us?" page.
The main downside is that the interest rates available are - as for government bonds - terrible.
Moneysupermarket has a couple of relevant articles:
An NS&I guide including this:
One of the main reasons for NS&I’s popularity is that it is backed by
the government, which provides savers with peace of mind that 100% of
their money is protected.
Savings held with other providers are protected up to a maximum of
£85,000 (as of January 2017) per person (and per banking institution)
thanks to the Financial Services Compensation Scheme (FSCS), whereas
with NS&I there is no upper limit.
And another one on savings protection generally with:
Savings invested with National Savings and Investments (NS&I) are
protected in full as NS&I is backed by the government. That means if
you have £250,000 invested there, it will be protected, so you don’t
have to restrict the amount you save to £85,000.
However, it’s worth noting that NS&I’s accounts don’t tend to pay the
most competitive returns, so stashing all your savings there just
because of the protection offered is unlikely to be a sensible