To give an almost diametrically opposed perspective to Nipvas, I have about an hour in a Gnat and two hours in Hawks. After the second Hawk trip I honestly felt that I could have safely taken it around the patch. It was CAVOK and about 15kts of wind straight down a nice long runway, and the Hawk just wasn't that intimidating to fly. I don't mean to claim I could have taken it low-level or anything tricky, just a few circuits.
The Gnat? I think I would've taken a lot longer before even thinking about going solo!