You should use bus lanes when they're available. Typically you can use the bus lanes outside peak hours. One of the advantages is that faster traffic can pass you on your offside (your right). It also keeps you further away from oncoming traffic.
You could fail your driving test if you choose to not use a bus lane when you could have. This could happen if you are asked to turn left and are not positioned to the left on approach to the turn (that is, if you cut across a bus lane you could otherwise have positioned in).
Also, if you choose not to use a bus lane and traffic begins to pass you on your nearside (your left) this could be marked as serious, as you should have allowed other traffic to pass you on your offside (your right).
Basically, drive on the left - unless you are overtaking or turning right.