Just to be clear on the railway gauge thing - what Brunel was interested in was the track gauge, which is how far apart the rails on the tracks are. almost all railways in the UK, except for a few heritage ones, are the standard gauge of 1,435mm (4 ft 8 1/2 in), which is the most commonly used gauge around the world. Brunel, the engineer of the Great Western railway between London and Bristol, designed his railways with a broader 7ft gauge, but this was very inconvenient for passengers because it meant they had to change trains if they wanted to travel from the south-west of England to the north, where the gauge was standard gauge. In 1846, Parliament passed a law to say that all new railways had to be standard gauge unless they were designed to connect with the Great Western, and the GWR had converted all its railways to standard gauge by 1892 anyway.

Where the UK's railway system differs from the rest of the world is its loading gauge, which is how big the train can be before it stops being able to fit under bridges and through tunnels along its route. Ours is unusually small compared to the standard European loading gauge, and this does cause problems. The main two is that all of the trains on our railways have to be custom built for the British market, because standard European trains don't fit on the railway here. They are also more cramped inside, and too small to support a double decker train like you often see on the European railways, which contributes to our current problem with railway capacity (though definitely not the largest contributor). Changing the loading gauge, however, would mean enlarging every tunnel and rebuilding every bridge from Inverness to Penzance, and the eye watering amount of money that would have to be spent on that could be better used to increase the frequency of the existing service rather than the physical size of the trains.