This coin has been overstruck and it looks official. If official, it is very rare. The undertype is not really visible, but it might be an IOVI type issued 321- 324, but since London is further than these might have normally circulated and pretty far outside of the territory of Licinius, maybe an earlier issue like a Sol type.
CNG description
Crispus. Caesar, AD 316-326. Æ Follis (22mm, 3.44 g, 6h). Londinium (London) mint. Struck AD 324. Laureate head right / VOT/X in two lines within laurel wreath; PLON(crescent). RIC VII 291; C&T 10.01.005. Dark green-brown patina, flan crack, overstruck on an uncertain issue of a larger module. VF.
