It is set out in S.I. 181 of 1997, Road Traffic (Signs) Regulations, 1997 that markings for bus lanes should be in white.
There are two main reasons why white is chosen over yellow for bus lane markings:
The first is retro-reflectivity which is a measure of the reflection of light and which is relevant when driving at nighttime. Higher values of retro-reflectivity can be achieved with white road markings than with yellow markings and therefore it is easier to see white markings at night.
The second reason is luminance which is a measure of the brightness of a colour. Higher values of luminance can be achieved for white markings than for yellow markings.
Overall white road markings have better conspicuity than yellow markings and It is for this reason that white is used for most road markings including for bus lane markings.