Goods are only classified under their constituent materials when they are not specifically referred to elsewhere in the Tariff. 

There is a multitude of goods made from plastics that are specifically covered elsewhere in the Tariff, for example:

- textiles and textile articles are covered in Section XI - for example knitted or woven polyester or nylon articles;

- footwear, headgear, umbrellas, walking sticks, whips or riding crops are covered in Section XII;

- imitation jewellery; 

- machines and mechanical or electrical appliances, parts of aircraft or vehicles are covered in Section XVII;

- optical, photographic, cinematographic, measuring, checking, precision, medical or surgical instruments, clocks and watches, musical instruments and accessories are covered in Section XVIII; 

- furniture, lamps and lighting fittings, illuminated signs or prefabricated buildings are covered in chapter 94; 

- toys, games and sports requisites are covered in chapter 95; 

- brushes, buttons, slide fasteners, combs, mouthpieces or stems for smoking pipes, cigarette holders, pens or propelling pencils are covered in chapter 96; and

- plastics that have been printed with motifs, text or pictures that define the goods (apart from floor, wall or ceiling coverings) are covered in chapter 49.

Source: https://www.gov.uk/guidance/classifying-plastics, July 24th, 2017