So, I’ll declare my hand: I work for the ISP in question.
The TTTV box is made by Huawei (though I have an older Humax test model).
It is a YouView box – that gets you (1) freeview thru the aeriel (2) access to BBC, ITV, Ch4, Ch5 on deman players. The key thing about YouView is that it gives you access to the last 7 days of TV on demand (assuming the channel has made it available).
With the TT service, (1) the box is free to all TT Plus customers (£14.50 per month + line rental) (2) there is various other content that you can only access as a TT customer. There are pay per view films. There are also other boosts – eg Sky Sports.
For the subscription stuff the difference is that you only pay on a month by month basis – want Sky Sports for the Ashes? Then only buy it for then.
I’m of course biased, but do I use it? Yes. And that’s a satisfied Sky TV customer.