Brompton can sell parts directly to the consumer. Here is the complete brake pad set you can buy on their online store. The length of the pads is 54 mm. The link points to the US website, but you can change the country to match yours (if available).
You do not need to buy the complete brake pad set (brake pads + brake pad holders) though. You normally only need to swap the brake pads (the rubber part) and get to keep the brake pad holders.
The brakes suggested by Brompton are from SwissStop. You could go to a bike shop and ask them for replacement pads of the same brand since you would want to reuse the pad holders. Brompton also suggests using the BXP series for their e-bikes for better braking power.
After changing worn brake pads, you usually need to readjust your brake calipers since the new pads will be much thicker than the old ones. You can try to give a bit of slack to your brake cable to increase the space between the brake pads and the rim by screwing in the barrel adjuster at your brake lever.