In a cafe that serves hot food, do I need a smoke alarm or heat alarm?

You may not need any, also if you did you would not normally be able to use domestic smoke alarms and heat alarms but instead a proper fire detection & warning system to BS5839-1:2012 with control panel ,manual call points, sounders and, where necessary, smoke & heat detectors.

If the premises are small enough for a shout of fire to be heard throughout you do not need a fire alarm unless you have a specific risk requiring detection (such as an inner room with no vision panel).

If the premises are larger they will require an electrical fire alarm system to BS5839-1 comprising control and indicating panel ,manual call points and alarm sounders (Category M system). If there is a specific risk where a fire would grow undetected such that it could compromise escape then you would need detectors as well (categories L5 to L1). Your Fire Risk Assessment would determine what Category is needed.

Due to false alarm risk heat detectors are used in and near to cooking areas.