I would add that a lot depends on the wheel offset you use. With the standard wheels, a 185 will very likely scrape at least one front wheel well (I'd be more definitive but we are talking about an early Lotus and some manufacturing tolerances, especially in the body, were somewhat loose.).
On the other hand, I believe our own EuropaTC, uses MGF wheels with 195/50-13 fronts and 205/50-13 rears without issue. The 205/50-15 tire has about the same diameter as a 185/70-13 so your speedo wouldn't need adjustment. If he doesn't reply to this thread, you should PM him to make sure I'm right.
There are advantages to going to 15" tires: they might be easier to find and the larger wheel allows for an easier rear disc brake upgrade.
Of course, if you want to stay with 13" wheels, tires are still available and if you have the Brand Lotus wheels, to my eye, they are one of the nicest aluminum wheels around.