A SIM-only deal which is genuinely PAYG?

[David Woolley]

If for any reason your account balance drops below zero, you may not be
able to access Three Services and you will need to purchase and activate
a Pay As You Go voucher or Add-on or otherwise make a payment to us to
clear the negative balance on your account.

It says "may", not "will", and says that negative balances are possible.

Whilst I suspect it might not be possible to completely avoid negative
balances, or doing so may limit roaming options, I suspect the OP wants
this for a child or for an elderly relative with poor mental capacity,
and wants as hard as possible a cap on over spends.

[The Natural Philosopher]

Post by David Woolley

If for any reason your account balance drops below zero, you may not
be able to access Three Services and you will need to purchase and
activate a Pay As You Go voucher or Add-on or otherwise make a payment
to us to clear the negative balance on your account.

It says "may", not "will", and says that negative balances are possible.
Whilst I suspect it might not be possible to completely avoid negative
balances, or doing so may limit roaming options, I suspect the OP wants
this for a child or for an elderly relative with poor mental capacity,
and wants as hard as possible a cap on over spends.

Well AFAIK my contact with Three does not include them taking money out
of my account without permissions.

They mayleyt you go slightly go negative, but they are unlikely to take
money off you.

But I am at the end of being 'helpfuil' Look the rest up your fucking self.

[Woody]

The OP should have a look at Pete Forman's excellent site
https://payg.pythonanywhere.com/
which shows all the PAYG rates available.