The Geico Gecko is apparently voiced by an actual British guy (currently it's some dude named Jake Wood). Who knew? It screams of horrible fake-ness to me. Like an American trying really hard to do an impersonation of Jason Statham's "london accent" impersonation in Lock Stock and Two Smoking Barrels/ Snatch. He's apparently a regular on Eastenders and had a part in The Illusionist. Somehow his Geico commercials are *worse* on the radio than on TV and I can't really pinpoint why. They just are. I wouldn't be surprised if he were asked to "cockney it up" for the part. At least that's what it sounds like.
Why on earth did they choose that accent for a gecko? Because Outback Steak House has a corner on the "bad fake Aussie" accents and to American ears I guess it's "exotic".