What does fully vaccinated plus 14 days mean in practice?

[candymad]

If I received my second jab on Thursday 8th July, am I able to return from an amber list country to the UK on Thursday 22nd July without having to quarantine? I'm confused about the plus 14 days, does that start from the day of the vaccination or the following day?

One day after 3pm 8 July is 3pm 9 July.
14 days after 3pm 8 July is 3pm 22 July.

So a difficult one, as you have identified.
If you had the vaccine in the morning, and could prove it, you might not have to quarantine. If you had it in the afternoon, you might have to quarantine.

Safest to plan that you might have to, but might get away with it.

Go for 23rd to be safe.
Depends on your vaccine too, AZ effectiveness looks to keep growing up to 4 weeks post vax.