Rising through the ranks
Well, there's one who did rather better than that. Wullie Robertson ended up as a Field Marshal and Chief of the Imperial General Staff, which isn't bad going for an ex-trooper. Off the top of my head I can think of one other too - from the nineteenth century. Hector Macdonald ("Fighting Mac") made it as far as Major General, but came to rather an unhappy end - committing suicide in Paris amid accusations of pederasty.
In the Navy it used to be easier - there used to be two types of sea officer - sailors and gentlemen (the problem supposedly being that "the sailors weren't gentlemen and the gentlemen weren't sailors"). This got harder in the nineteenth century, with the new requirement that an officer had to "pass for a gentleman", but there were two full Admirals of the Fleet, Sir Fairfax Moresby and Sir Provo Wallis, who started as Able Seamen (although in the case of the latter this was clearly a con, to get the young boy some sea time. He was entered in the books as AB at 4 years old, and was a midshipman by the age of 9). Perhaps even more impressively, Vice Admiral Sir William Mitchell not only started life before the mast but was supposedly flogged around the fleet for desertion. Just surviving this was a good effort - rising to the top of the Service was remarkable.
There is one other example that sprang to mind, although he didn't quite make it to the rank of General. This chap joined up as a private in October 1939, getting promotion to Lance Corporal a year later. He was then signed up for WOSB, where he passed top of his group. Posted off to military intelligence he rose quickly through the ranks and, by 1945 after service in Egypt and India, he became the youngest Brigadier in the British Army - one of only two men to rise from private to Brigadier during the war. A remarkable acheivement that, and one surely guaranteed to win the approval of the Guardian. What's that? Oh.