Do I self isolate? How long ago was it "safe" that I was with this person?

[BernadetteRostankowskiWolowitz]

Me and a friend had dinner together on Friday. We shared a taxi home (friend in from seat, out first, me in the rear, home second). Aside from the taxi we were 2m apart the whole time.

Friend has had a positive result today. They were not exhibiting any symptoms at all on Friday. Their dh was poorly but not with any Covid symptoms. Their dh tested positive on Tuesday (having subsequently developed symptoms). Friend tested yesterday with no symptoms (why!?) As she obviously lives with a positive case.

So.

How far back do contacts go? Do I need to self isolate?

She but if she is a mild/asymptomatic case she may be positive already; a test will not release her from the 14 days isolation but would allow her to start contact tracing her own contacts.
Remember the "who can get a test" is not based on the best system but the best way to use limited testing capacity, especially in England. That doesn't mean that is how it should be.

(My experience will be affected by the people I know in this situation being key workers, who are entitled to tests without symptoms if they have been in contact with a case)

I am cross with the government for the lack of testing for this reason. She shouldn't have got a test BUT she did and guess what.. it is positive. So now she can let her contacts know. Whereas she may have just self isolated and not tested meaning her contacts could also then have it without knowing.
Testing needs to be better. It is making mr so angry.