For your early flights casual clothing is sufficient. Your gear needs to be warm and wind proof. Most pilots quickly acquire a flying suit as a protective layer especially when ground handling and cleaning the aircraft. These can be acquired from pilot shops, military surplus stores, air shows or eBay. Expect to pay around £90.
Under your flying suit, light trousers and a T-shirt often suffices in summer. In the depth of winter you will need multiple layers to keep warm: a thick sweater, jacket and gloves - specialist skiing gear is quite effective.
The group rents "bone dome" helmets and you should take advice from an instructor before buying your own. Again these can be obtained from pilot shops, military surplus stores, eBay or indeed the manufacturers. New helmets can be expensive (over £1,000) surplus or second hand are much cheaper (around £500), but be careful to ensure you buy one with the correct microphone and earphones fitted and which is undamaged.
If you are a student pilot you will need to buy text books for the ground examination subjects and you'll want to buy charts, chinagraph pencils and a navigation slide rule by the time you solo. Expect to spend £200-300 on this.
If you have the budget for it there is endless scope to equip yourself with further accessories, including flying boots, kit bags, airfield directories and portable GPS systems.
For all these items instructors and fellow members will be able to advise you what to buy and the best places to get them.