American software doesn't need to be converted to be used on British computers.
Are you talking about the interface (the menus, dialogs, and panels) or about actual InDesign documents?
No its the sizes as in Edit > Document Setup like A5 - 34p11.528 x 49p7.276 rather than 148 mm x 210 mm ?
There are sizes like 3p0 and so on.
I need them to be in mms.
You can choose the "A" sizes in your North American version, and you can change your default measurements to mm in the prefs. Do it with no files open to make ithe default for new work. While you have no docs open, go to the Paragraph Styles panel, edit the [Basic Paragraph] style, and change the Language dropdown under Advanced Type to English: UK to use the UK dictionary for spelling and hyphenation in new files, too. It wouldn't hurt to also open the Character panel and verify the language dropdown there is also set to English: UK.
This is not the same as picking a dictionary in the prefs -- that actually has nothing at all to do with the language assigned to text, but is a way to select one of multiple dictionaries to use for a particular language when more than one has been installed. Language itself is a character-level attribute and you can have multiple languages in use in a single line of text (useful for spell checking foreign words). Unless you make a change, all new text or text you place that has no language assigned will automatically have the default language you've just set.