Have you tried to "flash" whilst driving?
This could be construed as not paying attention to the road,
Do you mean using a Libre reading to prove your BG was above 5.0mmol/l?
Whilst it is not 100% clear if this it is a legal requirement, the advice from DVLA is very clear that this must be a finger prick reading.
Do you mean your passenger has "flashed" your arm whilst you are driving?
I cannot see anything wrong with this unless your BG is too high and your passenger decides to inject you.
I would think it's okay if using the meter, but not the phone app; provided the meter is handy, the former doesn't need you to take your eyes off the road except to look at the result, while the latter would be too much of a distraction. I've seen police on TV drinking coffee while driving...