Air handlers and furnaces aren't typically used together in London. If you rely on a furnace, you probably shouldn’t need to think about an air handler.
Air handlers are usually paired with heat pumps and control airflow across your residence. Some units also provide backup 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. As furnaces rely on combustion chambers to make hot air, they don't need some of the pieces you'll find in a regular air handler.