You can find some hidden vehicles in the parks of London, such as a sooty van (Little Suzuki van with grass in the back) and a very fun golf cart parked behind huts and bushes around the parks
Select Free Roam mode once it is unlocked and make your way onto Oxford Street from the start point. Head west past the marble arch and onto Bayswater Road (use the map supplied with the game for road names). Follow Bayswater Road westward as far as the game will allow you to go, and then turn left at the blockade onto the Hyde Park Inner Road. The Hyde Park Inner Road U-bends back around to travel through the center of Hyde Park at this point. At the back of this U-bend is an open gate. Go through it, and parked outside a mansion is a Nissan Skyline Gtr and a TVR. In the garage opposite the mansion is a Lotus and a go-cart.
To get the Saab 9X concept car and a Triad Gang Car (souped-up Honda) in Free Roam mode, in the mission where Carter infiltrates Jolson's warehouse, go to Chinatown (located in Soho). Go down into the underground garage (the same car park where the Triads and Yardies have a gang war). The position of the cars varies on each floor.
To find a Tank in Free Roam mode, travel east along Birdcage Walk (near Buckingham Palace). There are some open gates on your right about half way along (just after the big white building on the same side). There is a small one-man tank on a plinth just inside the gates. You can shoot from the turret by pressing L3 (horn). It catches cars and people on fire.
To find a Lotus Esprit in Free Roam mode, drive west along Marylebone Road (above where it says Marylebone in red letters on the map). About half way along you will see a green dome on your right. Just after this is a small road on your right, which runs parallel to this road (it has Burger King and Vodafone shops on it). At the end of this small road is a subway entrance. Behind and to the right of this there is an alley next to the HSBC bank. The car is in this alley.
Another Nissan Skyline can be found in an alley behind some boxes in Bermondsey Street, near the Tower Bridge.
To find a Mk II (the car that you start in when you enter Free Roaming mode), can be found in the left hand side of a car park in St. Thomas Street, near the Tower Bridge.
In the "Speakers Corner" of Hyde Park (the corner closest to the Marble Arch) a Golf Cart can be found behind some bushes in an open grassy area. In car park of the building nearby is a Royal Parks gardeners pickup van, with grass and plant cuttings in the back.
To get a modified Peugeot 306, drive west along Horseferry Road (near the Lambeth Bridge). You will see a blue colored alley blocked off with some boxes on your right. Follow the alley and go down into the abandoned multi-story car park. At the bottom is a boy racer Peugeot 306 behind a large container. It has a bodykit, neon lights and a huge sound system in the back.
To get a Ford Transit, find one of the jumps around the city (for example, St. James's Street, near Green Park). There are Ford Transit vans parked in front of them.
Use the following trick to find the Go-Kart, Lotus, M250, Skyline, and TVR. The cars can only be found in free roam mode. Start by making a complete U-turn. Drive straight until you get to the T-intersection, then make a right turn. Go forward until you reach a four way intersection. Once again, go to the left. Drive perfectly straight until you see a large opening with a strange building with bars on it. Turn left, then take the first right turn. Next, drive perfectly forward. Go until the road splits (straight or right). Go straight. Soon, there will be a island with grass. Go over it and into an opening in the wall. Note: The Go-Kart and the Lotus are in the garage.
A Maserati Coupe can be found, only in the North American version of the game. In the same garage that you found the Modified Peugeot 206 is another sports car with stripes on next to it. This car looks closest to a Maserati, but its front resembles a Ford Mustang. Its performance characteristics are close to the other sports cars in the game such as the TVR.