The activities of Robert Clive and Warren Hastings were widely criticised at the time. Both faced legal action, Hastings in particular was impeached in a trial that lasted seven years - though he was eventually acquitted. Edmund Burke was Hastings' accuser.

Much opposition was practical, however. Burke defended the East India Company but believed that Hastings had encouraged corruption and waged illegal war as Governor General. You can see Debrett's record of the charges here. Adam Smith, on the other hand, argued that the British Crown had no right to grant exclusive commercial rights in another country and that this would "derange" the economy of both Britain and India (by taking stock/money out of one local system and pumping it into another) and supported the revocation of the EIC's charter. Smith is also generally dubious about the morality of appointing merchants to govern, recognising the obvious conflict of interest.

Moral opposition comes in two kinds:

1. some commentators claimed that "empire" (particularly in the East) would corrode the Protestant, democratic character of English people (and they do mean "English" - Scotland and Ireland's culture was often portrayed as authoritarian, and Wales ignored). You see this in say, Anna Laetitia Barbauld's poem "Epistle to William Wilberforce..." (1792): "By foreign wealth are British morals changed,/And Afric's sons, and India's, smile avenged." The abolitionist - and EIC chairman - Charles Grant (1746-1823) believed that converting India to Christianity would prevent this.
2. others recognise that the relationship between Britain and other peoples was exploitative. Now, the idea of "empire" here is not something much discussed in an 18th century British political context - the word is used very sparingly, America is a collection of "colonies" until 1783 (though people do refer to it as an empire) but India is seen as a primarily commercial venture (the co-option of Indian governments is a gradual process and this is usually what people mean when they refer to Britain "sleepwalking" its way into an empire). Burke describes Hastings' activities as predatory, strongly implying that we should view the EIC's exploitation in the same terms as sexual assault.

To come back to your original question then, Britain is not fully conscious that it is building an empire in the 18th century (though it is doing that) and discussion of the rights and wrongs of that is a 19th century phenomenon. Britain is aware that it is colonising and exploiting the world beyond its borders, and there is a well-established liberal and religious opposition to that - although it is mostly concerned with slavery in that period.