Although the reason why is pretty obvious.
Unlike other exotic carmakers, McLaren refuses to build an SUV, and has no plans to introduce an entry level model either. Clearly this hasn't stopped McLaren from growing, because the company posted a record sales year in 2017, following up a previous record year in 2016. In fact, following the release of new car registration data from the Society of Motor Manufacturers and Traders (SMMT), McLaren is now the fastest growing luxury brand in the UK.
The British sports car manufacturer registered 390 vehicles in the first half of 2018, marking a 47.73% year-on-year increase over the same period in 2017. To support all of those additional deliveries, McLaren added two showrooms in Hatfield and Leeds, and a third showroom on the south coast of England is also being considered. McLaren is hot right now, thanks to new models like the 720S and 570S. Sales are currently split almost evenly between the Sport Series and Super Series lines, and the company recently built its 15,000th car in just seven years. McLaren now produces 20 cars per day to meet global demand.
Obviously, McLaren is quite proud of its growth, and it is likely to continue to grow in the coming years. Still, it is fairly simple to explain why McLaren is the fastest growing automaker in the UK: it is also one of the smallest. It would be fairly impossible for a large company like Jaguar or Land Rover to increase sales by almost 50%.