There was a failure in a Virgin street box recently since when my speed has dropped from 500Mbps to 200Mbps even though my contract is M500
Could you access your router using http://192.168.0.1/ . Don't log in, select the 'Check Router Status' option and paste your 'Downstream', 'Upstream', and 'Network Log' tabs?
At least we can check the stats to rule out an issue with the line.
You can also check your connection speed in the 'Configuration' tab:
Primary Downstream Service Flow
Primary Upstream Service Flow