The two main types of solar Photovoltaic panels in use in the UK are Monocrystalline and Polycrsytalline panels You can also get a hybrid panel but these whilst higher capacity are more expensive per watt.
Monocrystalline Solar panels
The cells in these panels are slices cut from pure drawn crystalline silicon bars. The whole cell is aligned in one direction. This means that when the sun is shining brightly on them at the correct angle, they are very efficient.
These panels work best in bright sunshine with the sun shining directly on them. Monocrystalline panels tend to have blacker colour due to the fact they are absorbing most of the light that hits them.
Polycrystalline Solar Panels
These panels are made up from pieces of silicon moulded into a cell made up of several bits of pure crystal, rather than being formed from a single slice of silicon. As the individual crystals are not all aligned together and there are losses at the joints between them they are not quite as efficient as panels with the silicon perfectly aligned. Although because of this the cells work better with light at all angles, in low light.
The appearance is also different – you can see the random crystal arrangement and the panels look a little bluer as they reflect some of the light.
Which is Best
It may be that that polycrystalline panels are slightly better suited to the UK’s duller conditions, but the difference however will be marginal, as one panel might be more efficient in dull weather and the other more efficient in brighter light. So on balance the differences are marginal.