Originally Posted by GinFizz

Interesting - ExpertFlyer shows that as a Friday 06:00am departure - but that is clearly wrong. Thanks for confirming.

In that case regarding testing as I see it you have two choices:

Option 1:
Test #1 no later than 07:50 (e.g., 07:30 to be safe) on Aug 17th at the 04:00 - 24:00 testing-place (results back no later than early morning on the 18th);
Test #2 as early as possible (e.g., 08:00, just as it opens) on Aug 18th at the 08:00 - 16:00 testing-place (results back no later than 20:00 the same day - still more than 3 hours before travel).

[Test #1 < 48 hours before departure; Test #1 < 24 hours before departure; also more than 24 hours between Test #1 and Test #2, though in fact this is not actually a requirement if these are different testing institutions]

Option 2:
Essentially the same as Option 1 but paying the extra for the fast (2 hr) test result. In that case I would select more relaxed testing times, e.g.,

Test #1 around 11:00 on Aug 17th at the 04:00 - 24:00 testing-place (results back no later than 11:00 on the 18th);
Test #2 around 11:00 on Aug 18th at the 08:00 - 16:00 testing-place (results back no later than 13:00 that same day.

I can't tell you which of these to select. For Option 1 you will have both sets of results at least 3.5 hours before departure, which is plenty of time to upload all the documents. If that seems too short a window to you though, then Option 2 is probably better for you.

Regarding your other question (about whether to upload the flight details for the HEL-CTU flight when you apply for the Health Code prior to departing the UK, I don't know - but I would guess this is needed, and more importantly, it certainly cannot do any harm to upload these details. So I suggest you just assume that it is required, and upload copies of both the flight to Helsinki and the HEL-CTU flight.