Charcoal Barbecues are, as a rule of thumb, cheaper than Gas Barbecues. They are fueled with charcoal bricks, and as soon as they burn and turn white, this is the moment to cook. The cheaper price and easy operation makes it better if you are an infrequent barbecuer, and don't mind waiting longer for a Charcoal fueled BBQ to heat up.
The main difference between the two styles is heat source, with gas BBQ using gas bottles. By choosing gas, you often have more control over the heat, with many Gas Barbecues coming with dials to control each burner. This allows more precised and controlled cooking, as well as ignition at the flick of a switch, rather than waiting for Charcoal to white. Check out the Bahama Island Gas Barbecue, a truly premium product.
Propane is often the preferred choice of gas for UK barbecues, rather than butane which is used more for heating. Generally butane will burn hotter than propane, but a propane hose and regulator will allow gas to come out at a higher rate. Most Barbecues use propane as a heat source and bottles are readily available in the UK. Whereas BBQ such as the Rowlinson Calgary BBQ comes with hose and regulator included, be sure to check your BBQ to ensure the right gas type and regulator.