It varies. A lot of solar installers will push the benefits of microinverters or optimisers, because they can make your system cope better with partial shading or panel failure. However, the microinverters or optimisers are expensive, and can easily double the cost of the system. They also have dubious merit, because if you design your system properly in the first place it will cope with partial shading and panel failure quite happily anyway. However, selling the units makes sense for the installer, because it's more money that passes through their hands. Remember, the installers are not necessarily working for your best interests.

You can buy panels off Amazon at about £0.73/W. Add in the various bits that you need to go along with it, and that probably pushes it up to around £1.10/W, if you're doing it yourself.