The first Opium war was a total British victory. With such a total British victory, and with the expansionist ideals that existed in Britain, forcing China into an unconditional surrender seemed to be the the most likely action. I am aware that the British didn't have enough manpower to hold such a vast territory with such a large population, so Britain would not be able to topple the Qing Dynasty completely and set up another British colony, but it would have been possible to force China to cede, say, Tibet and Xinjiang to expand British India, and the major ports with the Pacific(Shanghai, Guangzhou, and such). This seems to be what the British would most likely do, to take from China all that it could realistically hold. The Chinese would comply with virtually any treaty, them being crippled by the British. Instead, the only territory ceded was Hong Kong and the island of Kowloon, which, at this time, was little more than a random rock. Why didn't Britain demand large amounts of territory from China?
I am aware that the British didn't have enough manpower to hold such a vast territory with such a large population, so Britain would not be able to topple the Qing Dynasty completely and set up another British colony
You answered it yourself:
- not enough manpower
- territory was too large
- too many people to control
The UK wasn't always looking for new territories. There were both a colonial and an anti-colonial factions in parliament. Sometimes the colonials were the majority, and sometimes the anti-colonials.
Prime minister at the time of the First Opium War was sir Robert Peel. He's remembered as the creator of modern police ('bobbies'). Sir Robert focused on domestic affairs, and was a proponent of free trade. Colonialism wasn't very high on his agenda, if at all. Colonizing China, not even remotely.
It's a myth that the UK (or any other colonizing nation) colonized whatever and whenever they could. Those countries have different political parties with different views on colonialism. Some are in favour, others firmly against. It simply depends on which party is in office.
Charles George Gordon (of Khartoum fame), for example, had the bad luck he was sent under an anti-colonial government.
Another example is the First Boer War in which Transvaal gained its independence. A pro-colonial government would never have granted it. The prime minister was Gladstone, who was [somewhat of] a liberal. Not a supporter of colonialism.