Ah, Cycling UK, those famous health and epidemiology experts.
It seems simple to me:
The second paragraph it's ok to carry onc cycling if you're not infected.
The third paragraph, if you're self isolating because you're either infected or been in close contact with someone who is infected, then you should stay indoors.
However...
The first paragraph is wrong. The advice for high risk groups is not to self-isolate, but to socially distance themselves. Self isolation is for people who are infected or who have been in close contact with someone who is infected.
Ignore any advice from me, or your neighbour, or Dr Nothing, the self appointed professor of bollocks who wants only to get his face on telly, and even CUK. Take such advice from the NHS website only.