Unsurprisingly, the amount of energy a solar panel can produced is also affected by the geographical location. Your latitude (top to bottom as you look at a map), affects the angle of the sun and therefore how much sun your panel will be exposed to, the intensity of the sunlight and also the optimum angle at which to mount the panels. As a general rule of thumb, the short-form optimum tilt angle calculation for the northern hemisphere is:

90 - [your latitude] + 23 (winter)

90 - [your latitude] - 23 (summer)

(For the southern hemisphere, subtract 23 in summer and add 23 in winter)

For the UK, London is at about 51-degrees, so the optimum tilt angles are 62-deg in winter and 16-deg in summer:

This calculation is an estimate, but should give you a fairly good idea about the different angles.