  We present 27 new L subdwarfs and classify five of them as esdL and 22 as
sdL. Our L subdwarf candidates were selected with the UKIRT Infrared Deep Sky
Survey and Sloan Digital Sky Survey. Spectroscopic follow-up was carried out
primarily with the OSIRIS spectrograph on the Gran Telescopio Canarias. Some of
these new objects were followed up with the X-shooter instrument on the Very
Large Telescope. We studied the photometric properties of the population of
known L subdwarfs using colour-spectral type diagrams and colour-colour
diagrams, by comparison with L dwarfs and main-sequence stars, and identified
new colour spaces for L subdwarf selection/study in current and future surveys.
We further discussed the brown dwarf transition-zone and the observational
stellar/substellar boundary. We found that about one-third of 66 known L
subdwarfs are substellar objects, with two-thirds being very low-mass stars. We
also present the Hertzsprung-Russell diagrams, spectral type-absolute magnitude
corrections, and tangential velocities of 20 known L subdwarfs observed by the
Gaia astrometry satellite. One of our L subdwarf candidates, ULAS
J233227.03+123452.0, is a mildly metal-poor spectroscopic binary brown dwarf: a
~L6p dwarf and a ~T4p dwarf. This binary is likely a thick disc member
according to its kinematics.
