Banks are cheapest -- period. The other factor is how much you change at once, since banks will generally give you a better rate for more money exchanged at a time.
These other places in Limeridge and anywhere else are rip-off businesses in a generally non-touristy city... They're thirsty to make a buck, because they're generally not exchanging high quantities, and that's all their business is. Banks do exchange in addition to a load of other services, so they're not trying to make as big a buck off you.