No comment on the legal aspect, but a few practical observations.
You need to think very carefully about the backup line which you use to tie the camera to the aircraft. The BMAA advice is good but is specifically for microlights with props at the rear end. In this case you will want to prevent damage to the prop at all costs. However, in a conventional powered aircraft (engine up front) or in a glider, if the camera comes undone, it can't hit the prop (well, maybe except in an aerobatics tailslide) so that's no longer a consideration. Airframe paint protection is: If you attach the camera with a backup piece of string or something, and it comes undone, then you will sit in agony for the rest of the flight while the camera makes a banging noise against the airframe and scrapes the paint off - or worse. If you cannot make the line long enough so that the camera dangles *behind* the airframe (and without the line interfering with the controls), then it might be best not to secure it at all - the cost of losing the camera will be significantly less than the cost of a respray. Put your name and telephone number on the camera and hope for the best if it falls off. Or don't if you're worried about liability.
And for the record, I have *never* *ever* seen a properly attached GoPro suction cup let go, unless it was attached to a non-airtight surface - in which case it fell off within a minute or so, so well before you even get the engine started. I have used mine on composite gliders, aluminium wings and perspex canopies at speeds well over 100 knots. I have had it attached to the underside and top of wings, and on top of the stabilizer and never experienced any aerodynamic problem.
Second advice is to treat the camera as a fire-and-forget item. Turn it on before the flight, turn it off afterwards, and don't think about it during the flight. The GoPro can do about 2.5 hours of HD video on a standard battery, and a 32 GB SD card can hold a little over four hours of video. That's plenty long enough for your average flight. Alternatively, put the camera in timelapse mode where it will snap a picture every second (or whatever interval you specify) automatically. The Wifi Backpack for the Hero 2, or the built-in Wifi in the Hero 3 Black Edition sounds great, but there is a huge risk of getting distracted.