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The link explains where you get them from.
We have them at home, as I am sure do lots of parents, for use by schoolchildren.
The problem with them is: they aren't as accurate as the PCR tests and this is more or less admitted by the fact that positive tests can be followed up by having to take a "proper" one.
I suppose as long as they aren't being posted out to everyone regardless, but given that they are, effectively, only a first stage in a process, then the whole thing does seem rather cumbersome.
If you test positive at school, by the way, rather than at home, then you cannot get a "second opinion" from the other test, which seems bizarre.