The finder of this coin, Martin Depear, said it was obviously Roman but that was all he knew about it. I was asked to provide a full ID and a valuation.
The coin is 20mm in diameter and has on the obverse a laureate head facing right, together with a legend reading IMP LICNIVS P F AVG. On the reverse, flanked by S P, the standing figure of Sol is depicted raising one hand and holding a globe in the other. The surrounding legend reads SOLI INVICTO COMITI and in the exergue are the mint letters (MSL) for London.
From the details given above I can say that this coin is a billon follis of Licinius I, whose dates are AD 308 to 324. In volume IV of David Sear’s Roman Coins and Their Values the coin is listed as number 15280 and dated to AD 316. It is also featured in the Standard Catalogue as number 718 and priced at £45 in VF condition.
Martin’s find would grade about VF and I’d price it no higher than £20. This is a good deal lower than the VF figure in the Standard Catalogue, which I regard as somewhat optimistic.