I went exploring last week, to take pictures of the north Lincs boxes before they are all abolished in the forthcoming resignalling works.
The IMG Reception Sidings box in blue can be seen by turning left off the southbound carriageway of Manby Rd A1173, onto a dirt access track,about 1/2 mile south of the roundabout where the A160 ends. You go over some gas pipes (I left my car near here) and then the track turns to parallel the railway and the box is 100 yds up on the right, across the lines. There weren't any No trespassing signs.
As to Immingham West Jc Box you have to drive into the port, amid the constant stream of lorries. Coming in on the A160, take 1st exit on the roundabout, go under the bridge, carry on straight along Humber Rd, through the port gate (nobody checked) and take the third exit on the roundabout onto West Haven way (see Google maps). This goes steeply up and over the railway. Straight on at next roundabout, and at the next roundabout, just after a level crossing, you will see the W Jc signal box straight ahead in some rough ground, strangely isolated from tracks.
Immingham E Jc I couldn't catch sight of. You carry on down the A1173 Grimsby direction, past Immingham village and turn L up Kings Rd which becomes Queens Rd. It climbs over a bridge over the line towards Grimsby, but the box lies back before the curve and wasn't visible from here. If you check the track diagrams you can get the mileage. It must be opposite the straight section of Queens Rd before the bridge, but there is thick undergrowth and it isn't visible. I tried following a track which led over gas pipes and the brook, but the rail fence was higher up, through thick undergrowth, and the box wasn't visible at all. If you have any success I'd be interested to hear about it!