Should I use the filing, publication, or grant date when citing a patent?
There is no clear single standard. Personally, I would use the filing date, because this date is most important when establishing prior art (which might invalidate the patent, or parts of it).

Anyway, for unique identification the patent publication number (which is GB2468195 in your example) is more important.

When targeting a journal, you might want to check whether there is anything in their guidelines.