I did start listing the evidence, and here is that now completed. Recent pictures have been found of two extant level crossing cottages on the Northampton-Market Harborough line that match the one in the OP. None have been found anywhere else that are anything like as good a match, so the location is assumed to be on this line.
I can find eight level crossings along this line in the NLS georeferenced 25" maps of 1899. All have a building of about the right size and shape to be one of these cottages, but most are on the wrong side of the railway (or road), i.e. mirrored in site plan. I don't think it matters whether we know why they were built, the fact is that they were. This is what is marked on those maps:
Little Bowden: mirrored, SB, SP away from road
(Clipston & Oxendon shown as a bridge, though the embankment is barely visible now)
Green Lane: mirrored, no SB, no SP
Draughton: mirrored, SB, SP beside SB
Lamport station: right, SB, SP beside cottage
Houghton: right, no SB, no SP
Spratton station: mirrored, SB, SP opposite SB and cottage
Merry Tom: mirrored, no SB, SP opposite cottage
Boughton: high skew, SB, SP opposite cottage (both behind SB)
Only at Lamport station is there a signal post marked in the right place. The two surviving examples are Little Bowden (post #83) and Boughton (#86 and #87). Both and the OP's are very similar if not identical in overall layout and roof form, and the very diagnostic features of the chimneys and barge boards. The front window and door porch are likely to have changed recently, but also vary in the older more limited views. Those items may have been changed early on, following damage (they all faced onto a road) or leaks. Boughton has decorative stonework at the gable eaves, hints of which are seen in other views.
However, while they all have the gable with a verge, only Little Bowden has the near-side "lean-to" extension with a parapet wall front and back. Also the detailing below the gable ends looks like brick, not stone. I'd say that looks like a different builder was employed for some reason, but working to the same drawing. However, it is a surprisingly major difference. The chimneys in the OP's example are a little shorter, too, which could also be one builder's interpretation of the plans.
So, lacking a picture that decides the matter, it does look like the OP's photo shows Lamport station crossing. It was also, I think, the only level crossing on this line with an A road at that date.