Forgot to add- notice on some days the curve is constant, but these are not the days with the highest UV- the spikier days, suggesting cumulus or other interruptions, reach the highest values; cloud cover or air type intensifying the UV?
The sides of cumulus clouds can reflect enough sunlight toward the surface so as to intensify the total input when the sun is also shining straight down on a location. This is why on quite cloudy days those few glimpses of sun can sometimes seem unusually intense for the time of year. The effect is most pronounced when the sun is close to, but not directly overhead; the angle of reflection is closest to straight down - so the sunlight travels through the least amount of atmosphere - without there being no reflection at all.
In the UK the sun is never exactly overhead so from around early March to late October we can potentially experience reflectively enhanced sunlight on any day with the right cloud conditions.
There is also some refraction of light through clouds - particularly high ones - that can get involved but at this point things start to get very complex, enough so that climate models struggle to even parameterise the effect, so I'll shut off my technical outburst here 
Oh and - I have achieved a very slight tan as of 11th March, which is as early as I've ever managed. This is thanks to several days this month with light winds, a good bit of sunshine and temps reaching the mid-teens. The March of the future? 
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https://twitter.com/peacockreports 2021's Homeland Extremes:
T-Max: 30.4°C 21st Jul | T-Min: -6.8°C New Years Day! | Wettest Day: 34.1mm 2nd Oct | Ice Days: 2 (27th Jan & 8th Feb)
Keep Calm and Forecast On