Show BSSID of an access point?

If I'm not mistaken, it should be the Address shown when issuing

sudo iwlist scanning

E.g.:

wlan0     Scan completed :
          Cell 01 - Address: 00:11:22:33:44:55
                    Channel:5
                    Frequency:2.432 GHz (Channel 5)
                    Quality=70/70  Signal level=-33 dBm  
                    Encryption key:on
                    ESSID:"the_known_ssid"
                    Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 18 Mb/s
                              24 Mb/s; 36 Mb/s; 54 Mb/s
                    Bit Rates:6 Mb/s; 9 Mb/s; 12 Mb/s; 48 Mb/s
                    Mode:Master
                    ...

If using network manager (the default in Ubuntu), you can use nmcli. It also has several options for the output format.

In your case, where you know the SSID and want the BSSID :

nmcli -f SSID,BSSID,ACTIVE dev wifi list | grep $wanted_ssid