Ive started noticing this in the last day or so in GL4 (Gloucester). Up until a few days ago, our download speeds never dropped below 350mbps
However as you have noticed there appears to be a trend in speed discrepancies kicking off recently. The speeds appear to ramp up again after a hub reboot, however soon tail off again. Possibly connected to Virgins roll out of their top 500mbps product and faster upload speeds for everyone else.
Im thinking some config issues at Virgins side are afoot