That depends on whether you use 'labour-only' or what's known as 'Bona Fide' sub-contractors. Labour only sub-contractors work under your direction and typically use tools and materials you provide ‒ and are paid by you or through an employment agency. Bona Fide sub-contractors, on the other hand, are paid by the main contractor but work under their own direction and provide their materials and tools.
You'll need employers liability insurance if you use labour-only sub-contractors – but Bona Fide sub-contractors are not treated as employees for insurance purposes.