Can they omit work from my contract and give it to someone else?

Generally no. An omission clause is the term that lets your contract be reduced by taking work out of it, and the courts read it as allowing genuine omissions, meaning work the employer no longer wants done at all, not as a device for stripping out your contracted work and handing it to another contractor or doing it with the employer's own people, unless the contract says so in very clear words. The reason is fairness: you priced the whole job expecting to earn from all of it, so letting the employer quietly reallocate the profitable parts would defeat the bargain. An omission dressed up as a way of giving your work to someone else is therefore usually a breach, and if it is, the measure of your claim is broadly the profit and contribution to your overheads you would have earned had you done the omitted work yourself. Read exactly what the omission clause allows, ask in writing whether the omitted work is being carried out by others, and price your loss rather than accepting the reduction. A reduction taken off your account is often called a negative variation, and you do not have to treat it as the end of the story.

Picture a landscaping contractor engaged to build both the hard paving and the soft planting for a new housing estate, with the planting the more profitable half of the deal. Partway through, the developer issues an omission taking all the planting out of the contract, knocking the value down as a negative variation. The contractor is ready to accept it until a site visit the following month shows a rival firm planting the very beds that were struck out. That changes the picture. Had the developer simply decided the estate did not need planting at all, a genuine omission, there would be nothing to claim. Handing the same work to someone else is different, and unless the contract spells out in clear words that the developer may reallocate work that way, it is usually a breach. The contractor writes to ask, in plain terms, who is carrying out the omitted planting, then prices the profit and overhead contribution lost on that work rather than swallowing the reduction. The answer turns on whether the work vanished or was simply given away.