Air handlers and furnaces aren't usually used together in London. If you use a furnace, you probably won’t need to think about an air handler.
Air handlers are usually paired with heat pumps and deliver airflow across your residence. Some units also provide extra heating and cooling components to support the heat pump.
A furnace works differently by blowing hot air into your vents, which send it across your home. Because furnaces rely on combustion chambers to make hot air, they don't use some of the components you'll find in a regular air handler.