Although it looks aesthetically better when upright, the battery in that position causes issues with the Brompton. Firstly it interferes with the fold as the seat will not drop fully. This means that if you have 'Eazy Wheels' fitted the seat tube will not fully engage and stop the bike rolling.
The folded bike is also bigger with the seat not going down so far and you end up with a huge lump sticking out the back of the bike (when folded). Finally, the power wires will inevitably end up at risk of becoming worn and broken as they must also fold repeatedly with the bike.
This Sparticle kit must have the smaller battery (at 24V) as it is the only one offered attached to the seat post. The 36V 10Ah setup offered by the Electric Transport Shop has the battery in a front bag connected with an umbilical (and also costs about 1200 quid for the kit alone!).
So; although a common place to put the battery on many electric bikes it is far from the best place for the Brompton.