Thank you for your enquiry.
We would advise that these panels are bonded to the roof directly. With the adhesive placed in strips across the panel/against the air flow. You do not need to ensure there is an airgap as the roof itself will act as a heat sink in high heat. The aluminium sheet also provides rigidity to prevent the panel from changing shape a previous issue with extreme heat and semi-flexible panels.
We do not recommend any particular type of adhesive as many of our customers use a wide variety. The brand name mentioned most commonly is Sikaflex which I think is available from Screwfix.
Normally photos of testing will suffice as we understand taking off the roof may be difficult once bonded.
The charger will be fine with these batteries and can charge them under the default setting it comes with, which is handy. However the kit comes with a dual battery solar charge controller, please note that if you are linking the two Halfords batteries together in parallel, these can be charged with only one of the battery terminals of the controller leaving a second terminal for charging your engine battery. Reading your later questions I think you are aware of this. Once one battery bank is full all the remaining charge is directed at the second battery bank.
When both battery banks are fully charged the controller will settle onto float charging mode whereby it will only trickle small amounts of charge to help maintain the battery at 13.8V. As soon as the voltage drops to 13.2V the controller will re-start the charging cycle to ensure the battery is fully charged again before settling back down to float charge. At 13.8V the controller simply does not allow any charge through almost like disconnecting the panels from the circuit as it were or water being held back in a tap, so no need to worry about excess power damaging anything etc.
The panels will still produce power in overcast conditions, you are right about season having effect. Right now for example you could expect maybe 60% of your panels output provided over a whole day!! So that would be about 90Wh per day or even less. However in summer even when overcast you could expect about 3-400% so maybe around 640Wh per day generally speaking.
Definitely parallel you also have to think about the controller when wiring is series as it would need to handle the higher voltage. Furthermore shading will also have less of an affect and generally the system would be more efficient.
Please review my response and let me know should you have any further questions or should you wish to proceed with an order.
Key Client Manager
Photonic Universe Ltd