We have identified eight to ten new cool white dwarfs from the Large Area Survey ( LAS ) Data Release 9 of the United Kingdom InfraRed Telescope ( UKIRT ) Infrared Deep Sky Survey ( UKIDSS ) . The data set was paired with the Sloan Digital Sky Survey ( SDSS ) to obtain proper motions and a broad ugrizYJHK wavelength coverage . Optical spectroscopic observations were secured at Gemini Observatory and confirm the degenerate status for eight of our targets . The final sample includes two additional white dwarf candidates with no spectroscopic observations . We rely on improved 1D model atmospheres and new multi-dimensional simulations with CO5BOLD to review the stellar parameters of the published LAS white dwarf sample along with our additional discoveries . Most of the new objects possess very cool atmospheres with effective temperatures below 5000 K , including two pure-hydrogen remnants with a cooling age between 8.5 and 9.0 Gyr , and tangential velocities in the range 40 km s ^ { -1 } \leq v _ { tan } \leq 60 km s ^ { -1 } . They are likely thick disk 10-11 Gyr-old objects . In addition we find a resolved double degenerate system with v _ { tan } \sim 155 km s ^ { -1 } and a cooling age between 3.0 and 5.0 Gyr . These white dwarfs could be disk remnants with a very high velocity or former halo G stars . We also compare the LAS sample with earlier studies of very cool degenerates and observe a similar deficit of helium-dominated atmospheres in the range 5000 < T _ { eff } ( K ) < 6000 . We review the possible explanations for the spectral evolution from helium-dominated towards hydrogen-rich atmospheres at low temperatures .