I'm looking for a lens upgrade?

Looking for an upgrade for nikon 18-55mm kit lens. Could do with a lower aperture and better optics. Budget about £500

My guess is that you want a replacement lens with a LARGER aperture. Lower indicates a small aperture like f/22.

Since you neglected to mention the brand of the camera system you have all I can do is send you to the Nikon and Canon websites. The lenses are listed with their prices (USD)

http://www.nikonusa.com/...index.page

http://www.usa.canon.com/...ens_lineup

Lower aperture doesn't make any sense, as it's ambiguous what that is. My guess is that you want a faster lens? Which means for an 18-55mm replacement, you'll be looking at something with f/2.8 constant aperture.

Since your budget is rather small for 2.8 zooms, you're probably going to have to buy a third party lens, from Tamron, or Sigma. Tamron's 17-50mm f2.8 is a reasonable optic, but it's focus is slow and noisy, however that's the best you'll get without spending £700.

The 17-50mm f2.8 will cover the range you have already but is outside your budget.

If you want a fast lens on a budget, consider the Nikon AF-S 50mm f/1.8. At £140, it is well within your budget. You will lose zoom capability as this is a prime lens.

Your other options are:

AF-S Nikon 17-55mm f/2.8 DX (£960)
AF-S Nikon 16-85mm f/3.5~5.6 DX (£450)

While the second lens is not faster than your 18-55mm, it is a higher quality lens - optics wise.