According to what I've read, Brunei is wealthy and is on the list of of "developed" countries.
Aside from the reputational damage to Brunei and its monarch, if economic/trade sanctions were widely applied, they would, to some degree, affect the country's economy. But we have not been at all reluctant to apply sanctions to much poorer countries when we don't approve of their politics - including preventing them from having access to vital medicines and medical equipment - so why not Brunei?
GG I would much rather have what you describe as "soft touch Blighty* than a regime that tortures, murders, disables and disfigures people.
Should women have equal pay and opportunities?