Hi Susier, I am sure that you ARE in the right place, and certainly not "in the way" at all.
I am on the airline side of flying, so cannot make any recommendations for your son in the Southampton area, but I am sure that other Ppruners will be along soon to help.
One thing I will say is that flying in a Cessna 152/172 might seem very very basic, almost toy like, no offence to Cessna. They are capable craft, (and I first learned to fly in one), but are built very light, and are cramped and, well, basic. Piper PA28s, (Cherokee or Warrior), are a little more like an aeroplane, if I can put it like that, and have four seats, although unlike the Cessnas, the PA28 has a low wing, impeding views of the ground.
30 mins is probably best for someone's very first flight in a light aircraft - there is a possibility they won't enjoy it, or feel sick etc.
Hope he enjoys it though !