Do people want to work in overseas branches?

You will have to ask these specific people to know the exact details. But in general, in the UK, the employer cannot force you to relocate unless you have a mobility clause in your contract and the relocation is reasonable. It's unlikely that the employees had a relocation clause to Malaysia right from the start, and asking an employee to relocate halfway around the world is probably not reasonable. So no, these people certainly volunteered to move abroad.

More generally, most (tenured) academics expect to be considered more like citizens of their universities than the usual work-related subordination relationship implies, and tend to want to have some agency in defining their working conditions and goals. In particular, this implies stable employment conditions unless the employee wants to change.

As for why they wanted to move abroad: there can be plenty of reasons. They want to form new collaborations with people from there. They want to strengthen existing collaborations with people they already work with. They want to take up the challenge of teaching to students in another country. Their pay is better compared to the local standards of living. They want to learn a new language. They want to discover a new country. Etc.

(PS: This isn't the 16th century anymore. The mindset of thinking "We'll open up an establishment in the colonies to spread our wealth and knowledge for the indigenous people" should be a thing of the past. There are simply smart people living and working in Malaysia, and the university of Southampton wants to work with these people.)

There are plenty of job adverts for positions at these branches. The job specifies the expected division of time between the two locations. Usually there are perks attached to the time spent abroad, such as higher salary or accommodation costs covered.

Expats that are sent/assigned from the home office are usually gloriously compensated - same/better standard of living as in the home country; international (read "prep") schools for the children; guaranteed number of returns back home; sometimes additional monetary compensation is tacked on for having to live in a country of certain levels of instability/danger - like "hazard pay". Usually you are at some elevated managerial level and probably hold an MBA or engineering degree.

There are some who simply don't want to uproot and leave their friends and families and their established lifestyles to a place where their native language might not even be spoken. On the other hand, there are those who would kill to be able to do just that. Probably the majority fall into the former category, hence the elevated perks, but what with international cell phones, email and Skype the world has become yet even smaller and more people are willing to uproot their lifestyles to exotics lands.