According to the force general orders issued for the landings: "cloth trousers, summer frocks and wicker helmets."
So undress frocks rather than dress tunics, at least according to orders. I think during this period the difference between the frock and the tunic was that the frock was a tad more loose-fitting and the tunic was more tailored. The frock had less buttons (five), too.
However, as Joroas says, the illustration in the Taiping Rebellion Osprey book shows a Tommy in the dress tunic instead. I don't know if this is a mistake or not, but it goes against the general orders.