Our van from Wicked Campers (www.wickedcampers.co.uk) would be a good choice for an extended family as it includes a two-man tent and sleeps three inside. For 730 pounds for ten days, it is pretty good value, especially when you consider it includes your accommodation (although do factor in the cost of a campsite if you decide to use one - typically 20 pounds or so for the night) and bear in mind these vehicles are pretty heavy on petrol.
The truth is almost anywhere in the UK is easily reachable and while the tradition may be to travel in such a way in the big open spaces of Australia and Canada, our little island is perfectly suited to this kind of trip. We are loving pulling into a layby and brewing our own cup of tea, but you could even pull into a pub car park (many will let you park overnight for free) and tucking into some pub grub and even a real ale before bedding down for the night. (The Good Pub Guide app is invaluable for this).