Three actively encourage heavy usage and allow tethering to PCs. They carry 40% of the UK's mobile network data and if they had a problem sustaining this they'd surely cap it, throttle it or abolish it. In fact they're expanding and promoting tethering and 'use all you can'. They don't throttle any more either, they tried to introduce it but users kicked off and they stopped doing it.
I live in a new build house miles from any telephone exchange, and cable isn't finished being finalised after being laid. Three gives us 25Mbps downstream most of the time and I schedule downloads for overnight, never had a problem.
For those saying heavy users spoil it, or are excessive, tell that to Three! Because not only do they not care, they explicitly promote it and allow it. The One plan rocks!
This was while simultaneously streaming live TV to a tablet, during night time hours (2 or 3am iirc). But as you can see, they easily provide solid speed and clearly have the capacity.
EDIT: For those saying 100GB a month is ridiculous on a phone... you have to remember they allow tethering to desktops, laptops, tablets etc. While I'd rather have a hard line (and will as soon as VM add our address to the database) Three condone and encourage this type of usage and it's easy to use those kind of figures in a multimedia rich household.