As a solo traveler, I stayed in a Hub (the one at Goodge Street in London) twice last year. The first time, I was in a Bigger room (that's what they call it), which would have been fine for two, if a bit small. The second time, I stayed in their regular size room. For one person who doesn't need a lot of space, it was fine, particularly since the room is very efficient (the space under the bed is huge - I put all kinds of things under there, in addition to my suitcases). But for two, I think the regular room would be unpleasant for more than a single night.
I'd definitely stay there again. I loved how bright and new the room was. I had a window, but since it was on a corridor that in turn had another window to the outside, I only got a bit of natural light. However, the room is very well lit, and you can even change the light settings from the Hub app! There's other tech fun, too - if you have the right kind of phone, you follow directions on the TV to put your phone's screen on the TV (very handy to watch movies I had loaded on my phone, on the much larger TV screen). And you can't ask for more outlets, even in a room many times the size (no trouble charging all your gadgets at once, and no difficulty keeping your gadget plugged in wherever in the room you want to use it, no matter how short your charging cord - even in the Bigger room).
My credit card wasn't accepted by the website, so I had to pay on arrival, which went fine. And I liked the breakfast a lot; there's oatmeal in addition to breads, juices, pastries, fruit, etc. The breakfast does not have meat. For my first visit, I did not pre-buy breakfast, and it was easy enough to buy it on the days I wanted it, just before eating. For my second stay, I knew I wanted it every day, so I bought it at the time I booked the room.