There are two model/spec options for the newer Macbook Air:

So it’s an extra £500 to upgrade the processor from 1.6GHz to 1.86GHz and 120GB SATA drive to 128GB SSD drive. However, if you click to configure the cheaper £1,299 machine and take it to the same spec as the £1,799 machine it actually works out CHEAPER!

You can see from the screen shot above that to upgrade from the 1.6GhHz to the 1.86GHz costs an additional £140. To upgrade from the 120GB SATA to the 128GB SSD costs an additional £350. So when you add them both (£140+£350) you pay an extra £490 which is £10 cheaper than buying the pre-configured machine with exactly the same specifiation.

Starting with the cheaper model and upgrading the processor and hard drive takes you from £1,299 to £1,789:

But many people assume that a pre-configured bundle must be cheaper, otherwise what’s the point in having it as an alternative option, but it looks like you are paying Apple £10 to click the upgrade buttons for you because it costs £10 more:

This only seems to apply to the UK version of the Apple online store, I just checked on the US online store 

which shows the model with the faster processor and SSD as $700 more than the base model. If you configure the cheaper version to the same spec as the more expensive version:

You end up paying $200 for the processor upgrade and $500 for the upgrade to SSD so a total of $700, the same as the higher spec pre-configured model.