When I last looked some time ago, the OS recommended using Grid In-quest
http://www.qgsl.com/?product=gridinquest
Which they say can be downloaded free, and with no restrictions on use.
They also claimed that the calculations should be accurate to ~ 10cm.
It can convert OSGB36 -> ETRS89 - which is essentially WGS84 adjusted to
remove movements of the European Tectonic Plate - which should be less than
50cm difference.
Most OSBG -> WGS84 web algorithms I've seen seem to suggest that the
calculation should be good to around 3-7m
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