The dress policy at our swimming pools is that any clothing worn in the pool should be safe, clean and modest. Within these parameters you are welcome to wear what you feel comfortable in. Some women like to be covered up and wear leggings and long sleeved tops in the pool and others will prefer many different styles of swimsuit. Men too will have many different preferences.
As long as clothing is not excessively baggy as this type of clothing can fill with water and drag you down, and as long as your swimwear is clean and sufficiently modest (any standard two-piece or swimsuit as a minimum for women and standard fitted or loose trunks and shorts for men), then you are good to go.
Also, check out this guide from Swim England with some suggestions on what to pack when going swimming.