Alas, the main problem is not enough sunlight in UK winter.
Been there, done it, and failed, simply because the 1 Ah per day of Mazda3 (or ND) dark current drain is more than the maybe 0.5 Ah of charge from the panel on a rare bright sunny winter day inside the clean windscreen, or a more usual 0.1 Ah on a dull overcast day with a dusty windscreen.
This was with a borrowed “10 Watt” panel after quickly giving up on my 2.5W Noco. The 10W panel was twice the area of the 2.5W but still only produced a miserable current - no risk of boiling the battery!
I used the OBDII connector inside the car, faced the car South and angled up the panel in the windscreen to maximise the insolation.
It would be a different matter in the tropics where the sun is a lot stronger and for longer.