Try downloading something like the 50MB file . . . .
The 50Mb file took 20 seconds to download on its own. I also downloaded the 50Mb file at the same time as the 100Mb file and the 50Mb file then took 30 seconds to download.
20 seconds for 50Mb works out (I think) at 20Mbps. I immediately followed this with a thinkbroadband speed test which indicated 15Mbps, then a BT speed test which indicated 32Mbps.
The thinkbroadband speed test result (15) seems reasonable given the manual download test (20). I don't believe the BT speed test (32), at that speed I think the download should have taken 12.5 seconds.