Lets see...
I would think there would be some interesting repercussions with Britain being at war with Russia during the Napoleonic period, as they were both fighting France (granted, Russia was nominally an ally of France for awhile).
As for the future, with Iran an ally of Britain, a European war will be interesting. Now, Russia probably doesn't like them much, but probably dislikes the Ottomans more. So they won't really mind having someone around to open up another front against the Ottomans. Of course, this would occur to the Central Powers and they'd probably try to keep Britain (and thus Iran) out of the war.
But if Britain and Iran enter, the Ottomans fall pretty hard (unless they've been doing better for themselves). This helps out the Russians as the logistics of their situation improves greatly, now that allied shipping (and maybe some expeditionary forces?) can reach them easily.