The tbbx1 looks more like congestion, and without giving detail the tbbx6 by requesting the data multiple times can work around around congestion.
Most testers provide one figure so you are oblivious to many things. A handful report a figure that would be not dissimilar to the burst figure.
Difference between the burst and average, on a connection where software and connection are behaving they should be almost the same. In your case burst looks to have been affected probably by AV/firewall software.
So currently result looks like some congestion on the Sky network
There is a broadband for dummies, but this sort of stuff is invariably missed out as they devote pages to bits and Bytes, url, hyperlink etc