I am amazed that Tony didn't mention this, but the BBC micro had a number of expansion connectors under the keyboard facing towards the front of the machine. To access these ports, you had to lift the whole machine and plug IDC terminated ribbon cables. As I recall, you had the 1MHz expansion bus, the printer port, the Tube (2nd processor bus) and a disk interface connector. You also had a power output connector allowing you to power external devices (e.g. external floppies).

From left to right, 34-pin floppy interface, 26-pin parallel printer,