If something happened exactly 2 weeks ago...

[lanbro]

...is it within 14 days?

So if I tested positive on Tues 28th Dec then want to travel on Tues 11th Jan, is that within 14 days, or is it 15 days? Am getting myself very confused!

I'm not sure where you are that you have to do 14 days, but the UK way of counting is that onset of symptoms or day the test was taken is day zero, and the time required runs until midnight at the end of the final day.

So I make the final day of the 14 as 11th and freedom day 12th

The self isolation guide explains this

If you are notified by NHS Test and Trace of a positive test result you must complete a period of self-isolation. Your isolation period starts immediately from when your symptoms started, or, if you do not have any symptoms, from when your positive LFD or PCR test was taken, whichever test was taken first. Your isolation period includes the day your symptoms started (or the day your test was taken if you do not have symptoms), and the next 10 full days.
This means that if, for example, your symptoms started at any time on the 15th of the month (or if you did not have symptoms but your first positive COVID-19 test was taken on the 15th), your isolation period ends at 23:59hrs on the 25th of the month.

www.gov.uk/government/publications/covid-19-stay-at-home-guidance/stay-at-home-guidance-for-households-with-possible-coronavirus-covid-19-infection#SymptomsPositiveTest

It's not for isolating, I'm out of isolation, Spain say you can't enter if you've tested positive in the last 14 days...I can't decide if from Tuesday 11th counting backwards whether 28th December is within 14 or 15 days!

I agree with Roseandrose. I saw something similar at the airport when I went on holiday in July that makes me think it's probably the same rule. Someone had been vaccinated 2 weeks before (to the day) and they weren't allowed to board until the following day as day of vaccination is day 0.