There are a few interesting things hiding behind the undertray so let's get it opened and have a look.
You are going to need an 8mm socket to remove the 5 screws
In the centre near the back you will see this screw.
Just infront of the rear jacking point on both sides you will find a screw.
And another one each side about 18' forward of the previous screws.
With those five screws out, the tray will fold down from the centre.
The tray cannot be removed with the smart sitting at normal height,
if it must be removed it can be forced or the rear of the car will need to be jacked up.
You are going to need an 8mm socket to remove the 6 screws.
The first two screws are found in a crater behind the front wheels.
The second 2 are found in the back of the wheel arches.
The last 2 are found near the front of the wheel arches.
With those removed the tray will swing down from the middle.
There is just enough room to slide your body in and lay on, working on the car isn't
impossible although the restrictive area can make it hard to do fiddly jobs.
The front tray needs to be dropped down to access the brake light switch box and horn.
Do I Need To Remove The Front Panels?
No. Just remove the bolts from the undertray, feel where the undertray joins the front panel.
Put your fingers in, pull down to release the two tabs and pull the front panel out.
The front panel has just enough flex to be able to clear the under tray to allow it to lower.