[decimon]

I am unaware of any dialect or variant of the English language, possibly excepting 'ebonics', that uses anything other than 'millennia' as the plural of 'millennium'.

Certainly not 'British' English.

Try some dictionaries and argue with the publishers.

[SAJ]

Cite some.

I have my old Union Standard Dictionary (London, 1955) right in front of me.

[SAJ]

Interesting. The American Merriam-Webster’s 2nd International (aka, ‘the unabridged’), Springfield, MA, 1961 does offer ‘millenniums’ as a secondary plural.