Is it possible to make binding "promise to purchase" contract?
Party A will provide services to party B, on the condition that party B will be obligated to make a purchase of goods from party A within set time frame. The purchase will be based on specified prices.
Will I require a witness to overview the signing process?
I would prefer to deal without them.
On general contract law principles, you can enter into a contract that specifies you will buy thing X for £Y within period Z. The terms of the purchase need to be reasonably specific, so that it isn't just "an agreement to agree". The key terms will identify X, Y and Z in my example: what will be purchased, for how much and when? Depending upon the circumstances, however, there could be other key terms.
The execution of this sort of contract would not usually need to be witnessed for it to be effective.
I have assumed, here, that this is a B2B contract and doesn't involve any unusual or especially regulated goods or services, or land etc.