What would have happened to the British economy if.....
.....Napoleon hadn't lost at Waterloo?
....the American civil war hadn't happened?
....the Boer War hadn't happened?
....the Scottish highland army during the Jacobite rising of 1745 hadn't turned back at Derby and had marched on and captured London, putting the Stuarts back on the throne of Britain?
....the Indian 'mutiny' (arguably a war of independance) hadn't happened?
....the Opium wars didn't happen?
Shit when you look at it history is just one armed robbery after another
From the above i would probably go with one of the first two. The war against France saw the British navy emerge in control of the seas, whilst other European navies were destroyed and it saw Britain avoid bankruptcy, whilst other countries which had also borrowed heavily lost credit. Also the British victory at Waterloo may have stopped the spread of French revolutionary ideals, allowing the rich to continue weilding power in Britain and drawing up policy based around maximising their own profits.
The American civil war cut the supply of cotton to Britain which was a big industry at the time. Britain stayed out of the war though because the north supplied Britain with much of its grain. Britain did however sell arms, machinary and warships to both sides in the conflict.
These two 'what ifs' would be fairly straight forward to research and pretty interesting to study as well. Niall Fergusson made a good series on channel 4 (see 4 on demand) called 'the ascent of money' which covers some of this stuff...good luck!