If you want to block out external noise, you're really looking at getting noise-canceling specific headphones, least you want to go deaf from playing music too loud and never be able to properly hear music again.
With noise canceling headphones, you're either looking an in ear monitors (aka canalphones), or noise canceling headphones. If you travel, or move around with listening to music a lot, I recommend IEM.
I'm the pricing of headphones in pounds, but some brands to look at are, Shure, Etymotics, Ultimate ears, Weston. Sennheiser, Koss, V Moda, amongst others also make IEM, although I haven't used all of them.
I'm not too sure about noise canceling headphones, because I use open back headphones (having IEM as a noise canceling solution), but it wouldn't be hard to research that. Sennheiser, Sony, Audio Technica, I'm willing to bet make some, most likely in your price range.
If you specifically want headphone that will play loudly, look at the Ohm spec for the headphone, the lower the number the easier the headphone is to drive. Headphones with a higher number won't be able to be powered by a ipod's sub-par amp.
If want you headphones louder and more importantly to sound better at all volumes, you can look at getting a portable amp, to bypass the ipods one (which I already said is subpar). But it's above your price range, and I wouldn't recommend getting an headphone amp, unless you have headphones that are good enough to benefit (i.e. are very detailed and accurate, like high end IEM), or won't work without the extra power (high ohms).
Hoped that helped.
Also don't get skullcandy's like one of the people replied. Their are **** sounding (objective), and have poor build quality (objective) and make my eyes bleed their so ugly (subjective)