If you want a fun and cheap way to see England's West Country including Stonehenge, Bath and Longleat I can thoroughly recommend camping. We stayed in a "ready made tent" at a campsite called Brokerswood Country Park near Westbury. It cost about £150 for three nights in a tent that could sleep up to 8 - shared between 8, the cost would be an absolute bargain. All mod cons are provided in the tent - stove, electricity, cooking equipment. Although has to be said the camp beds were a little on the small side. If you're heading down by car from London you could stop off at Stonehenge on the way.
If you're going to Longleat I would recommend buying a "passport ticket" and giving yourself 1 and 1/2 - 2 days there. You can then visit all the attractions at your relative leisure.
The above campsite provides easy striking distance to Bath as well as the beautiful Wiltshire countryside, famous for its engraved White Horses on the Chalk hillsides.