Gilts - what are they backed with?

Dear Sir or Madam,

I understand that the UK treasury raises funds from foreign and
domestic investors using 'Gilts'.
Could you please tell me, as succinctly as possible, what a 'Gilt'
is and what it offers as it's 'security' or surety to the
purchaser.
For example: If a domestic consumer takes out a secured loan with a
private bank then the device used as security will be a tangible
asset such as a house or car. The sum borrowed is thus 'guaranteed'
for repayment to the bank as it now holds a charge over this
physical asset. What is the equivalent security for a government
issued 'Gilt'?

Yours faithfully,

Andrew: Smith

Your ref: 9/546

Dear Mr Morran,

Many thanks for your letter explaining how Gilts are traded and why
it is that investors can buy them with such confidence.

I understand from your answer that the 'security' for any
prospective purchaser is simply the fact that the UK Government is
a trusted issuer of such instruments. 'triple A rated', so to
speak.

May I ask what incomes the Government relies upon to be able to
offer this absolute surety of payment of interest and principle on
these bonds?
e.g. Is this 'surety' or 'rating' based on the fact that the UK
Government can prove the level of funds it is likely, (or
guaranteed?) to receive in a given period by collecting Taxes due,
fines and the like, (by way of the Government's own various
Departments) based on the number of UK citizens of working age?
i.e. It's probable or guaranteed income?

Yours sincerely,

Andrew: Smith

Dear Mr. Morran
Nicely sidestepped, but I think you have answered my question...

Would you happen to know which credit rating agency(ies) deals with
'UK plc' and 'determines its 'rating'?

Yours sincerely,

Andrew: Smith