An interesting article posted by someone above here...
http://www.virginmedia.com/corporate/media-centre/press-releases/virgin-mobile-launches-the-uks-firs...
...which includes the following hopeful, though slightly confusing line at the bottom...
"Truly Unlimited is the UK’s only unlimited plan with no tethering, throttling, speed restrictions or caps"
Yes, that's not a well-formed sentence and is kind of ambiguous. Well, strictly speaking an ambiguous sentence is one which could mean a number of different things, and this sentence has ONLY ONE possible meaning. It is clearly not meant to be a meaningless sentence, such as an over-zealous sales person or a politician might use. If it is intended to mean anything at all, then it is intended to mean that Truly Unlimited has none of the usual restrictions that one might expect, and has no tethering restrictions, no throttling or speed restrictions, and no data caps.
The sentence certainly DOES NOT mean that tethering is not allowed. In the context, that would be an absurd interpretation.
But what a struggle it is to get the definitive answer from Virgin Mobile! On the phone to Virgin Mobile, so far I've been told by two people that tethering definitely IS allowed, and by two people that is definitely IS NOT allowed.
(The most plausible answer I've had is from a Virgin Mobile employee in the Cancellations dept who says they were recently on a training course, and it was made clear that tethering is ONLY allowed on the Truly Unlimited deal, and not allowed on any other package. The reason he gave for that is that on limited packages, customers can easily exceed their allowance on tethering, be charged extra, and then because Virgin have no record of what kind of data was used (e.g. did they download 10 movies or were they streaming Spotify for 18 hiours a day) they can't demonstrate what data was used to justify their bill. But with Truly Unlimited they will never be in that situation.
That kind of makes sense. Like every other network provider, Virgin of course has to try to ensure that people don't use too much data on the 4G network, but Virgin's safeguard is that fact that they only offer this Truly Unlimited package to people who already have an unlimited home internet service in the form of Virgin's own home broadband. So, the logic is that no one will do a lot of tethering because the Virgin fibre broadband will be faster and less hassle anyway.)
I've asked to see the relevant Terms and Conditions, but so far have not found any mention of tethering anywhere in those documents, and neither have any of the Virgin Mobile people I've asked. I'm on the phone now to try to get the definitive answer before signing up.