A Microgeneration Certification Scheme (MCS) accredited supplier will design a Solar PV installation to the Government approved standard (SAP 2009) and will supply the details of the design as part of their quotation since it underpins the financial analysis for the annual return on investment etc.
You can do your own calculations however by using this web site.
As can be seen in the screen shot on the left all you have to do is to use the google map facility to pin point your location and fill in some basic parameters in the form provided.
You can select a PDF output and then the "Calculate" button will produce your design, as illustrated on the right.
This will not only show you the likely annual total electricity production but also the likely average daily output for each month of the year.