The fitters on this page say they will install a kitchen you have bought from somewhere else, such as a trade supplier or a national retailer. Several name the suppliers whose kitchens they fit regularly.
Agree in writing what the fitter's price covers before the kitchen arrives. Ask whether it includes taking out the old kitchen, fitting the worktops, plumbing in the sink and appliances, tiling and removing waste. Check who does the gas and the electrics: anyone who connects a gas hob must by law be on the Gas Safe Register, and electrical work in a home comes under Part P of the Building Regulations. Ask the fitter to check the supplier's plan against your room before you place the order.