I am not familiar with Staverton but have had my PPL in the past and flown out of Biggin Hill airport so know a bit about flying. If it is about a one off experience it really doesn't matter too much which you choose. If you are able to do a bit more checking then the flying club that has the more experienced instructors, not just a lot of through traffic on its way to commercial ratings would be the better bet. From my experience if they have an instructor who also has other 'bows to their arrow' such as being a radio telephony examiner etc the better. If they have a CFI rating (Chief Flight Instructor) that shows the flying club has the clout to have someone with a higher level of experience and can actually sign off its students. Ask questions about what is actually included in the trial flight, do they give you a log book (a nice to have momento but will form the basis of any flying they go onto do in the future) or charge extra and that will give you an idea of how much they want to make out of you or how much they want your custom going forward. You shouldn't need to pay for extras like headset hire but every club is different so it might be worth checking. Ask about the aircraft that they might be flying. A Cessna is quite common, as are a Piper PA28 or a PA38 (both low wing, four seater and two seater respectively). You might get better visibility from a Cessna but they are horrible in anything other than calm air and from my experience make you feel as sick as anything (maybe that is me, but I have had others say a high wing does that to you).
I also wouldn't recommend anyone other than the instructor flying with the person getting the experience. It may sound nice but is generally a huge distraction. It also depends on the aircraft available as whilst many light aircraft are four seaters, some are tiny two seaters.
Good luck and bear in mind it is an expensive hobby and you may be complicit in someone taking up a hugely expensive hobby. I have forgiven my Dad for indulging me once every two months and my school for allowing it as a 'sport' (three weeks of two hour aircraft cleaning equalled half an hour of flying once a month!)
. You can start flying at a young age and the hours you clock up from aged 14 doesn't help with the arguments. DD at 11 is already learning her flight navigation and it is helping her maths no end, at present she doesn't want to fly herself, only solve the faster than light equation which seems more feasible to me that my being able to afford two of us flying.