Lat/long transformed to OSGB has an offset when plotted - cartopy

Question

If I take a set of lat/long points, convert them to OSGB crs using cartopy's "transform_points" method and then plot them, they are offset compared to plotting the lat/long directly. If I transform them back to lat/long, they plot ok. If I convert them to UTM coords they plot ok. Am I missing something?
Example code:
import numpy as np
import matplotlib.pyplot as plt
import cartopy.crs as ccrs
from cartopy.io.img_tiles import OSM
# set up figure and background map tile
fig = plt.figure()
ax = plt.axes(projection=OSM().crs)
ax.set_extent((-2.473911, -2.410147, 50.567006, 50.605896))
ax.add_image(imagery, 14)
# Plot some test points using lat/long (PlateCarree) crs
test_lonlat = np.array([[-2.464482, -2.432523, -2.437892], [50.593243, 50.596390, 50.573177]])
plt.plot(test_lonlat[0], test_lonlat[1], 'r:+', transform=ccrs.PlateCarree())
# Transform to OSGB coords and plot
test_OS = ccrs.OSGB().transform_points(ccrs.PlateCarree(), test_lonlat[0], test_lonlat[1])
plt.plot(test_OS[:, 0], test_OS[:, 1], 'kx-', transform=ccrs.OSGB())
plt.show()
Thanks for any advice.
Simpler example using single point
fig = plt.figure(figsize=[7, 7])
ax = plt.axes(projection=OSM().crs)
ax.set_extent((-2.435, -2.405, 50.575, 50.595))
ax.add_image(OSM(), 14)
# Simpler test of OSGB crs
# Take point at end of outer breakwater: OS (370783, 76226) or lat/long (-2.414343, 50.584978)
point_os = [370774.0, 76221.0]
point_lonlat = [-2.414278, 50.584971]
# Plot each on OSM tile - both look ok but small error
plt.plot(point_os[0], point_os[1], 'r+', transform=ccrs.OSGB(), markersize=15)
plt.plot(point_lonlat[0], point_lonlat[1], 'kx', transform=ccrs.PlateCarree(), markersize=10)
# Convert lat/long to OSGB and plot - now offset by ~50 m to NW
point_os_new = ccrs.OSGB().transform_point(point_lonlat[0], point_lonlat[1], ccrs.PlateCarree())
plt.plot(point_os_new[0], point_os_new[1], 'm^', transform=ccrs.OSGB(), markersize=10)
# Print both sets of OS coords
print(f'Original point: {point_os}')
print(f'Transformed point: {point_os_new}')

Answer 1

When you create an axes to plot with this line of code
ax = plt.axes(projection=OSM().crs)
you are expected to use cartopy.crs.Mercator projection coordinates to plot on it as OSM().crs is Mercator.
This part of your code:
# Transform to OSGB coords and plot
test_OS = ccrs.OSGB().transform_points(ccrs.PlateCarree(), \
test_lonlat[0], test_lonlat[1])
plt.plot(test_OS[:, 0], test_OS[:, 1], 'kx-', transform=ccrs.OSGB())
uses OSGB coordinates to plot on Mercator projection. That's the wrong part.
The correct lines of code should be
test2_OS = OSM().crs.transform_points(ccrs.PlateCarree(), \
test_lonlat[0], test_lonlat[1])
ax.plot(test2_OS[:, 0], test2_OS[:, 1], 'kx-', transform=OSM().crs)
Edit1
The tile images in my plot above is OSM(), via ax.add_image(OSM(), 14).
The problems the OP faces is not using matching coordinate systems between the data and the axes, or wrong values.