It also depends a little on what you mean by "roaming" around characters. DLR doesn't have the underground system of tunnels like WDW, so you'll often see characters walking through the parks on their way to the M&G location. It's not necessarily that they're roaming (they're heading somewhere), but it can look like it and you'll often see people run up to them. They usually don't stop, but sometimes they'll let the family's walk with them and take photos. The Stormtroopers in TL do truly roam; they just wander around, so you'll never know where you'll find them!