That Halfords one looks way too small as well - we're only a family of three but are looking at at least a 5 man tent to get a reasonable amount of space. The Outwell website has a useful little detail when you look at tent specs - how many the tent will sleep comfortably rather than just the industry standard (60cm width per person). The bedrooms in that one would just fit two single airbeds, for example, and the person furthest from the door would have their face mushed into the sloping ceiling. We currently have an alleged 6 man tent (no living area) and it fits three single airbeds with about 10" gaps between them!
It's really worth having a look at tents pitched in a showroom, even if you can't find exactly the one you want it should give you and idea of what the sizes are like in reality. We're currently dithering between the cheaper option of a Hi Gear Mojave 5 and the more expensive Outwell Montana 6 (which MN reckons I really ought to have, by all accounts). The Hi Gear is £200 (was £130 a week ago, but DH wouldn't let me buy it until we'd taken the cheapo tent out once more) and the Outwell £500. Yesterday I decided on the Hi Gear, leaving £300 to spend on carpet, footprint, better camping furniture etc, and now DH isn't sure any more. I LOVED the Outwell in person, just not sure it's worth 1.5x as much ... but then I woke up at 4am today thinking 'what if DD wanted to take a friend with us one day, would the '2 man' bedroom in the Hi Gear be big enough?' :o
Toronto looks interesting - looks like a range Outwell only do for Argos, with lower specs (lower waterproofing rating, and if you compare the Youtube video of pitching a Montana with the one for the Toronto you can see how much thinner the poles are). You can see it still has nice Outwell touches though, and they have such a good reputation I'm sure they wouldn't risk it making a crap cheapo range.
If you like the style of the Halfords one (personally I don't, I think they feel claustrophobic) DB and SIL are getting a Hi Gear Zenobia 6 for themselves and three dogs when the price comes down shortly (Go Outdoors are dropping it from £230 to £150 in a couple of weeks apparently). Gets good reviews on UK Campsite.
Oh, the thing I liked best about the Montana was the light roof - the Mojave is blue and quite dark inside, while the Montana has a cream roof is all light and lovely. Definitely not something to pay another £300 for though!