One UK store with a decent choice is vapeclub (mentioned in an earlier post).
Over 3500 flavours from 250 vape brands.
They also have a sale section with around 50 juices and 40 shortfills for the moment.

An other UK store with a large choice (around 300 brands) I know of is rainbowvapes.
They also have a cheap e-liquid section.

An alternative to ready made liquids are shortfills.
They usually make the resulting liquid a bit cheaper.