The United Kingdom government does not deny the exchange of votes between it and the Saudi Arabian government, which, alone, may not be a big deal. However, the United Kingdom does have a significant influence in global affairs and has many allies that could also have been a part of the deal (that were not exposed as the United Kingdom was).
It is highly likely the UK was influenced by Saudi Arabian oil and other resources in exchange for votes to the UN Human Rights Council. Saudi Arabia's poor track record with Human Rights is under greater scrutiny in light of 47 executions over the weekend. With such turmoil and reputation it is likely Saudi Arabia must have sweetened the pot to receive such influence.
UK's government was instrumental for getting Saudi Arabia voted onto the UN Human Rights Council. With the current situation in the Middle East, they were aware of nurturing an alliance with a somewhat friendly country in the area to help combat the rise of ISIS, Iran's growing ambitions and other terrorist threats in the nation.
Basically, when was the last time the UK did anything important on a global level? Probably back when they were colonizing places left and right, and even then they didn't do much good. Kind of funny that they'd be trying to get Saudi Arabia on the UN Human Rights Council because a few decades ago they probably would have wanted to rule them.