Full OSGB Grid Reference with two-letter prefix:
Go
Numeric OSGB Grid Reference with selected prefix:
HP HT HU HW HX HY HZ NA NB NC ND NF NG NH NJ NK NL NM NN NO NR NS NT NU OV SC SD SE SH SJ SK SM SN SO SP SR SS ST SU SV SW SX SY SZ TA TF TG TL TM TQ TR TV Go
(Set default 10km square by bookmarking a page of the form /?defsq=OV)
Fully numeric OSGB Grid Reference (Easting and Northing; normally 12 digits):
To obtain a grid referrence for a feature / point on OSM, I would recommend this site.