british english - -ise or -ize in IELTS writing

There are many places suggest not to mix British and American spelling in IELTS test, such as the articles on IELTS-Blog and nativespeakeronline (No link here. I don't have enough reputation to post more than 2 links).
When I search on forums, the most common idea is people in North America prefer -ize and people in other places prefer -ise. However, an article on learnenglish.de says:

the Oxford University Press insists that words such as computerize, capitalize, capsize, organize, organization, privatize, publicize, realize should take the -ize ending

And for some words in this list like "organization", people on forums say most British still prefer "organisation".
In this case, if I stick to using British spelling on IELTS test, should I use -ise or -ize for the words in this list?

Answer

I'm an American and use American spellings for everything. This answer is based on comments, including a British English speaker, and internet research.

Most British speakers and publications use -ise endings. However, as you have pointed out, Oxford Spelling uses -ize.

This has led to some British English publications and speakers using -ize, while most ignore the guideline and continue to use -ise. Tellingly, the London paper The Times switched from using -ize to -ise in 1992.

Therefore, if you're using British spellings for everything else, you should probably use -ise, but shouldn't be penalized for using -ize.