Have a feeling that this is a long way from regulatory approval. Interim solution (that I'd use) is a sealed lead acid battery such as you can find on the
CPC Web site. Some quick calculations (based on a 4:1 Rx:Tx ratio) based on the stated current figures indicate that average current drain will be 0.144A. Assuming a maximum flight endurance of 5 hrs and adding 100% safety margin means that you should be looking at a 12V 2.2Ah model, such as the BT0267266, which wieghs 1.0 kg and measures 60x34x178 mm (H * W * D). One of these costs £6.62, so should hardly break the bank. Downside is that you may want to buy 2, so that one can be on charge and the other in use on the aircraft.
Just a thought.