it's likely that the majority of 'smallish' drinking glasses from the C19 and much of the C20 have some degree of conical ism to the shape of the top of the foot, and of these most will have a flatish underside - it may simply be aesthetics and/or tradition. As mentioned by Jeff, historically - so the explanation goes - such glasses had originated with high conical shape to the their feet to avoid the pontil scar from scratching the table, though the days when a conical shape was essential were long gone even by the year 1800. For my money too, feet with a conical appearance always look better than flat feet.
To say .................... "That foot is conical so probably from the earlier part of 19th century" ……….. is misleading - not deliberately of course, and the glass in question may well have that sort of age, but that's to ignore the fact as I've said, that a big proportion of well-made glasses over the past two hundred years will frequently show some degree of a conical foot.
I seem to recall we chatted about folded feet some few years back, and the fold - possibly Italian crystallo in origin - was created to strengthen the edge of the foot, rather than having anything to do with pontil scars, in the same way that some of the Murano houses created folded top rims. In the U.K., the folded foot made a temporary re-appearance some time c. 1820 - 1830, and of course it's been a Murano feature for much of the early and mid C20 on some of their impossibly thin creations.
IIRC, the reason why first half C18 drinking glasses were left with high domed feet - and the untreated scar - was more due to a lack of technical inability to properly remove the scar - thus you needed a high foot. When they did begin to achieve this c. 1760, then the high domes were phased out, although some degree of dome remained.
Sorry - can't offer anything worthwhile as to a possible date for this piece - it may well not be British.