If you are on a Crop, and your kit lens doesn't quite cut it for you, and you want a faster wide angle lens, the bottle neck becomes your rather small budget. Wide angle lenses in general, and good quality fast ones in particular, are notoriously expensive.
If you are hard set on Canon EF lenses, one of the lenses that would somewhat come close to the £150 budget and is a wide angle lens would be the older EF 24mm f2.8 in the used market. It is still a decent lens, of course recently superceded by the f2.8 IS version. On your crop it would not be really wide, but will be wider than the 50mm. I think the older EF 20mm f2.8 is a tad more expensive and is not great on image quality front compared to the 24mm f2.8. I occationally dust off the 24mm 2.8 as it is one of the most compact lenses in the line up before the pancake came along. The new 40mm pancake, while cheap, will not be really all that wide compared to the 50mm on a crop body. The other option may be the 28mm f1.8. We will not even go near the 24L or other wider primes or zooms given the budget.
Even for the used Canon EF 24mm f2.8, or EF 28mm f1.8, you may need marginally a few more dollars ...I mean pounds. Even if you can garner £50 more, and get up to £200 total, on the used market you can easily grab one of those or get a third party lens.