Skype as web traffic?

[#1]

I tend to use Skype around the same time every day.  Sometimes the video quality can be very good, other times bad.
I've noticed that the bad picture seems to correlate with my Web usage increasing, whereas on the good days my Broadband phonecalls traffic increases.
I assume the poor quality is due to differences in how Plusnet prioritise the different traffic in the evenings.  Is there anyway of blocking Skype from acting as Web traffic so it is always prioritised?

[#2]

Answered here.
It's line speed on all account types - i.e. never restricted.
Are you sure you are not maxing your upload bandwidth which would cause degradation in quality.