TNA Steed's Cars
TNA Jaguars and Rovers
1. He's driving a "Big Cat", a green Jaguar XJ12 Coupe 5.3 litre, an automatic model.

It was a road-going replica of a Jaguar Broadspeed.

It's licence plate is NWK 60 P.

First appearance of the car is in House of Cards.
Steed's Jaguar XJ 12 Coupe
Site with many Broadspeed photos and they mention John Steed.
Well, Steed maybe an excellent driver (as seen in Invasion of the Earthmen and Death's Door), but he isn't an excellent parker.
"I never believed in rules". (Steed in The Danger Makers)
2. 1971 Range Rover, with licence plate TXC 922 J.

John Steed's green 1971 Range Rover in Target!
Steed destroyed the car on purpose to prevent the launch of a rocket in Obsession.
First appearance House of cards Steed driving for the first time
Rovers

1. A yellow Rover 3500 SD1 Automatic with the licence plate MOC 229 P.
It's the first car of Steed that appears in The New Avengers in The Eagle's Nest.
[Damaged cars][Home][Which Car When?]
During The New Avengers Steed is not longer driving any vintage cars, but a powerful Jaguar and two Rovers.
Nevertheless he still owned a vintage Bentley, as the car gets destroyed by a bomb in Dead Men Are Dangerous.

The Jaguar and the Rover 3500 SD 1 can be seen in his garage in Hostage, a 1971 Range Rover stands in another garage in Cat Amongst the Pigeons. Steed is driving a few other cars during the series, but these three cars belong to him.

Jaguar
In August 2015 new information about Steed's 1976 Jaguar XJ12 C was published.
The car will be in an auction of classic-auctions.com. The website informed that the auction will take place on 14th October 2015. Besides several photos of the car in it's current state several details about the car got published:
Reg Number: NWK 60P
Chassis Number: 2G1008BW
Engine Number: 7P5411SA
Cc: 5343
Body Colour: Green
Trim Colour: Tan
- John Steed's famous mount in 'The New Avengers' TV series

- The eighth XJ-C 12 made and originally a Jaguar experimental car (as confirmed by its Heritage Certificate)

- Broadspeed body kit and extra wide wheels fitted for the TV series plus respray in Metallic Green and re-trim in Beige leather etc

- Dry stored for many years and now in need of restoration

source: www.classic-auctions.com
Description of the car on website in pdf
several photos on website:
www.classic-auctions.com
The Jaguar in August 2015.
Photos by classic.auctions.com
On 14th October the Jaguar that was offered by  H&H Classics and has sold at auction for £62,000 - £50k over pre-auction estimate of £10,000-12,000.
source