Most builders you engage will likely have b-layers, carpenters, and plasterers on staff. You'll need plumbers and electricians at various points. If building work is held up by any of the trades you engage directly, you may have to cover the costs of that lost time.
A building firm may well be able to secure more competitive quotes for these other trades than you will yourself, to confirm you'd need to source quotes for both options.
To cut down on initial costs, you could specify a builders finish where the final touches such as flooring and decoration are excluded and you look after them yourself as time and finances allow.