Sorry if I am confusing the reading of these graphs here!
If you assume the uv scale starts at zero , then the first green band covers zero to 1.0, second green band 1.0 to 2.0 and so on.
i.e all the uv numbers are rounded up, does this not make sense?
I think you are right as UV indexes can be inbetween for example 1.3C or 4.6, and that if it reaches the top of the defra number then it is indeed that number.
Also those talking about the met office forecast I've come to suspect these are rounded to the nearest whole, and also only represent the top likely index for that day based on whichever run they are derived from. The past 2 days down here in Portsmouth have also had 7's in the forecast but in reality the highest recorded at at a local private station was 6.3.
Also defra have more detailed graphs available but apparently only for certain stations and upon request that give indications of how accurate the automated graph are as human input is put to use