Opposition to imperial practices in Britain did exist in the 18th century, particularly with regards to the East India Company, but it was more about challenging how imperialism should be practiced, rather than whether or not it should exist at all. Many objected strongly to the practices of the East India Company, but did not oppose British imperialism in general. This viewpoint was promoted by some very prominent British politicians and writers, most notably Edmund Burke. Burke and his allies did not reject British imperial rule in India or elsewhere, but they did present an impassioned attack on the brutal and corrupt practices of the East India Company. In the process, Burke and others presented a powerful critique of imperialism, even though they did not advocate the end of overseas empire. Burke’s position was most notably expressed at the impeachment trial of Warren Hastings, the former governor-general of India for the East India Company, which began in 1787.

India, and the practices of the EIC and its employees, had been a major political issue in Britain for decades before things came to a head in 1787. British victories in India over Indian kings and the French in the 1750s and 1760s vastly expanded the amount of wealth and influence possessed by the EIC. With no meaningful limitations placed on them by the British government, the actions of the EIC became more and more brutal as they leveraged their new power to extract wealth from India. In the 1760s, company officials forced farmers in Bengal to switch from growing food to growing opium for export, causing a massive famine. In the Wealth of Nations, published in 1776, Adam Smith sharply criticized the policies of the EIC, particularly those that caused the Bengal famine. Granted, Adam Smith was using the Bengal famine as a case study for the dangers of excessive state interference in the economy, but in the process he produced a critique of imperialism as well.

Corruption and insider trading by company agents made some men fantastically wealthy, allowing them to return to Britain with enough money to buy enormous country estates with cash. Stories of atrocities in India, distrust of men who returned from India with fantastic wealth, and most importantly, a financial crisis in the company caused by spectacular mismanagement lead to parliament stepping in for the first time in 1773. The new law established the post of governor-general of India, which would be held by an EIC official, as well as a council of other company officials he would report to in India. In theory, the law established a degree of government oversight of the company, but in practice the company continued to operate without oversight in India. Warren Hastings, a long serving EIC official was appointed as the first governor-general. Following a series of major defeats of the company’s army at the hands of Indian and French forces in the early 1780s, new concerns emerged about the EIC. After enormous controversy and the collapse of one government, the India Act of 1784 was passed, which created a governing board in London that would have regulatory power over the EIC. The rhetoric being used by MPs both in debate and in the law itself was at times, very critical of the practices of the EIC. The law even went so far as to state “to pursue schemes of conquest and extension of dominion in India are measures repugnant to the wish, the honour and the policy of this nation.” This statement would not be upheld by the people on the ground in India, but it does reflect the fact that very real concern and opposition to EIC practices existed in parliament at the time. Some of this was simply a reaction to recent military setbacks in India by company forces, but there still was real opposition to the actions of the EIC.

Many of the attacks against the company in 1783 and 1784 were directed at Warren Hastings, the governor-general of India. He returned from India under a cloud of suspicion in 1786, and formal impeachment proceedings began the next year. In the impeachment trial, the arguments made by Edmund Burke went significantly further then earlier attacks. Burke spent four days making his opening remarks at the trial accusing Hastings of “fraud, rapine, and violence,” and would continue to lay more charges against Hastings over the course of the trial. Most notable for the purpose of this question is how Burke discussed the people of India in his prosecution. Burke explicitly stated in his first speech that the nobility and princes of India were just as noble and ancient as the British nobility. This speech was given before the house of Lords, making this a rather remarkable statement. Other arguments made in his opening speeches reflect sympathy for the people of India. Burke speaks at length about the outrages committed against the farmers of Bengal. Burke attacks Hastings for overseeing the famine in Bengal, and for excessively taxing farmers to destitution. Burke also shows at least some degree of respect for Indian traditions and customs. When he attacks Hastings for allowing the sale of certain offices of government, he points out how this practice has violated matters that are “held most sacred among the people of India.” He doesn’t elaborate on that matter, so it may just be a rhetorical flourish, but later when talking about the caste system, he states “We are not here to commend or blame the institutions and prejudices of a whole race of people, radicated in them by a long succession of ages.” This claim was made in contrast to Hastings’s defenders, who claimed that Hastings’s actions were justified by an obligation to correct the defects in Indian culture, such as (in their view) the caste system. While the 18th century caste system cannot be defended as an equitable or positive institution, British efforts to “reform” the system were generally self-serving policies that only made matters worse for most Indians.

However, despite this enlightened seeming rhetoric, Burke was still not really anti-imperialist. One can scour everything he wrote or said without finding any blanket condemnation of empire. He did not demand that Britain withdraw from India, he only objected to the methods and practices of the EIC. In this way, Burke and other 18th century critics of the EIC certainly differed from later anti-imperialists, who rejected the whole system. Many of Burke’s arguments, such as the claim that the Indian nobility was no less noble then British nobility, sought to extend quasi-European status to India. He brought in large maps to the House of Lords to illustrate his comparisons between India and Europe. Burke claimed that India had all the requirements of civilization, with kings and aristocrats and well developed states. This approach allowed Burke to attack the abuses of the Indian people and the nobility of India without requiring him to repudiate a racial hierarchy that put Europeans on top. When Burke spoke about Africans or the Indigenous people of the Americas, he did not grant them the same respect he did Indians.

Warren Hastings was ultimately acquitted by a large majority of the House of Lords in 1795. The trial became a public sensation and a political mess, and it dragged on for years. Burke’s sensational arguments attracted a lot of attention at first, but over time, interest waned, and so did concerns about brutality in India. In the 19th century, the arguments made by Burke largely vanished from mainstream political discourse in Britain, which obscures the great amount of controversy over imperial development in 18th century Britain. While British imperialism as a concept was never seriously challenged in the 18th century, the form it should take was hotly contested. Colonial ideas that would become codified in the 19th century were, for the most part, still under development in the 18th century, and many key aspects of later imperial policy faced serious challenges before they became common practice.

To give a final answer to your question, yes there were major voices in 18th century Britain that sometimes quite strongly opposed “‘forcibly civilizing’ and/or wiping out other people and then extracting their wealth.” However, despite this fact, 18th century opponents of these practices were quite different then later anti-imperialists, and cannot really be called anti-imperialist themselves, since they did not oppose empire.

Sources:

A New History of Britain Since 1688, Susan Kingsley Kent (chapters 2 and 4)

“Rethinking Burke and India,” Daniel I. O'Neill https://www.jstor.org/stable/26224074

The Works of Edmund Burke - Volume 10 https://www.gutenberg.org/files/18192/18192-h/18192-h.htm