There is no avoiding that testing colours in situ is an essential part of the journey of choosing colour. All 191 shades in the Little Greene palette are available in 60ml tester pots.
Many site-specific factors will affect the way colours appear - the two main ones being lighting conditions and adjacent colours.
With this in mind, it's a good idea to test in more than one location, applying two coats over as large an area as you can. Observe them in daylight and artificial light, the difference can be quite significant. Avoid positioning small swatches of several colours immediately next to one another - each will affect the way the others read.
Also, try to avoid comparing the swatch to bright white as this can be misleading.