Once upon the time, the British Empire was almighty and ruled over the 7 seas. Secret of the Third Reich is not about this story.
In 1949, England is a battlefield of Horrors. Nuked, London is now a forbidden place, grave of Churchill and its cabinet. Bombed, the Southern part of the island is a no-man's land, where it is usual to see zombies, victims of the V-Gas.
Gibraltar fell, and the underground fortress is theater of the most horrible fights. Egypt may fell anytime if its defenders are not vigilant and lose faith in the fight. In the East, the Indies are under pressure by the Soviets and the Jap's.
In 1949, the United Kingdom is fighting to the very last man against what is perceived as the Truest Evil on Earth, the Junker's, the SWD and sometimes the Soviets. British units are brave and have seen all.
British units in SOTR have a common rule to re-roll all their Horror tests and field the most unit with "Monster Hunter" skill. They are the Nemesis of the 3rd Reich.
There are 2 official OoBs for Britain, Rifle and Para. Rifle is a common infantry platoon unit while the Para are comparatively expensive but good.
British have good support options, not highly armored though, they focus on speed and flexibility. British Steel are formidable opponents and Para Sniper (in the support options) are the best Sniper even if not clearly required. A great guide to UK option can be found here.
More on the Sniper. While they have a lot of skills that allow them to start first and be anywhere on the board, this is usually not a key asset as most sniper rifle have no limit meaning they are highly efficient even if fielded in the original deployment area.
On the background, the British player can play both Good Evil (The Temple and its deadly secrets) or the True Good guys (the M13). British can fight both Germans and Soviets. It is still possible for them to fight against the US, but this looks strange unless you field a Temple unit, then it will make sense.
In strategy, the British seems more expensive and less armored than their opponents. The Para OoB is symbolic of this trend and the lack of heavy armors too.
Against Soviet the British can face issue against an all-Mecha Platoon, the Brit lacking heavy armor and Mecha/tanks.
They are my preferred army. They have the right balance and are not black or white. And while they lack heavy armor, they balance it with true courage.