There's a 1000 tonnes triple expansion steam engine (the world's biggest working one) in west London. They run it about 7 times a year (Kempton steam museum).
Oh , I knew that, just wanted to say there's an enormous steam engine here. There are two, but the second is rusty and doesn't work. There should have been three but by the time the second was built it was cheaper to replace the third with steam turbines (much cheaper to buy but more expensive to run). The other steam museum in the area is mostly beam engines, including a "Bull" engine (no beam), a working 90", a non working 100".