What is the difference between an option and a right of preemption?

In Brown v. Gould, [1972] Ch. 53, Megarry J. said at p. 58: Under an option, only one step is normally needed to constitute a contract, namely, the exercise of the option. Under a right of pre-emption, two steps will usually be necessary, the making of the offer in accordance with the right of pre-emption, and the acceptance of that offer. The failure to provide either a price or a formula for ascertaining the price is accordingly far more serious in the case of an option than under a pre-emption: he who exercises such an option may well be virtually signing a blank cheque, whereas he who is entitled to a right of pre-emption can at least refrain from accepting the grantor's offer if the price be too high.