Hi ANCO,
What you wear for flying depends on how much flying you do, what you fly, where you fly (eg, arctic? tropics?) and what kind of flying you do.
For the average UK PPL, flying two hours a fortnight in a Pa 28, street clothing is all you need. Just make sure you wear shoes with a definite sole and heel, not a through-sole so that if you have to step onto a stirrup-style foot hold, when entering or leaving the aircraft, you don't slip and injure yourself.
Open cockpits are cold and draughty and the draught sometimes blows up your trouser leg so dress accordingly. An expensive sheepskin Battle of Britain style jacket is all very well as a pose, but little use when there is an icy blast blowing up your leg and freezing your family jewels.
If you do a lot of flying, especially say over open country in Scotland in the winter or over water, then the RAF advice 'dress to survive' (on the ground or in the drink) makes good sense. Heed it.
Similarly for the high annual hours fraternity, a lot of nonsense is talked about fabrics. Natural fibres aren't just natural fibres when it comes to fire. Silk and wool are animal products and will shrivel when burned, but cotton and linen are derived from plants and will BURN LIKE A BL**DY TORCH and are thus best avoided.
In regard to man-made fibres, again there are good and bad. Most, such as polyester (and polycotton), will melt onto your skin and then burn (as the Royal Navy discovered in the Falklands). While Nomex II and Aramid fibre will not burn. However, don't fall into the trap of thinking that Nomex II or Aramid will render you fireproof. They don't; their only advantage is that they don't burn (or melt and burn), but neither does a flying suit made from a single layer of these fabrics protect you from being burned when wearing them.
BP.