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?