After being pointed in the right direction by origimbo's link to the National Insurance Fund Accounts for 2021, I think I've done enough research to answer my own question.
I asked because this question reminded me that many journalists and commentators in the UK regard National Insurance as an outdated stealth tax with the implication that NI payments go into the general taxation pot. This is not the case. There is indeed a separate National Insurance Fund:
The money is held in the National Insurance Fund (NIF), separate from
the Consolidated Fund. Contributions are not "taxes" because they are
not directly available for general expenditure by the government.
The Consolidated Fund is the main bank account of the government. General taxation is taxation paid into the consolidated fund and general spending is paid out of the Consolidated Fund.
The CIPP has a brief history of National Insurance which states:
The National Insurance Act 1911 formed the basis of NI as we know it
today, albeit on a considerably different basis. The fundamentals,
however, remain unchanged, as both employees and employers have always
had to pay NICs and the NI Fund always granted employees entitlement
to certain benefits.
There were two individual schemes at that point in time, one dealing
with health and pension benefits and the other associated with
unemployment benefit. The former was run by trusted societies and
unions whilst the latter was a scheme controlled solely by the
government. This soon changed with the arrival of the ‘welfare state’
in 1948, which heralded the homogenisation of the separate stamps
resulting in one singular stamp to cover all benefits.
in 1975 the stamps became redundant as contributions were
no longer paid at a blanket flat rate that was applicable to all.
Instead, NICs (national insurance contributions) were calculated based on the level of earnings an
individual received and were collected via PAYE, at the same time as
The income of the NIF consist of contributions from employees, employers and the self-employed, plus interest on its investments. The NIF are used to pay for social security benefits such as state retirement pensions, but not for the means tested Pension Credit and Tax Credits. National Insurance contributions also provide a small part of the funding for the public healthcare systems in the UK however the Government determines the total allocation for health each year.
According to this House of Commons briefing paper,
In every year since 2009-10, payments from the National Insurance Fund
have been greater than contributions made. As a result, over the last
five years the Fund’s balance has shrunk from a £50 billion peak in
2009, to around £22 billion today (May 2014). The latest figure is equivalent to
25% of annual benefit spending.
The CIPP claims that
the state pension is predominantly funded by ‘live’ NI contributions
So clearly, the NIF is not the sole source of benefits or health spending. I'm not sure if that was the intention of the Beveridge Report, I guess that's a different question.