Well, we've just rebooked our April itinerary to August due to... you know... that situation...... Originally, we (family of 5: me, husband, mom-in-law, 15-yo daughter, 14-yo son) were going to spend a week in London and then fly to Granada and hang there for 4 days, mostly to chill out and eat our heads off (it wouldn't have been our first trip to Granada; we just love being there). But now that we are rescheduling to August, we'd rather stay in England with the cooler temps.
So.... We will fly in to LHR on a Monday in August, arriving late evening. We'll crash at the Premier Inn at the airport that night. We then have 3 or 4 nights that are totally unscheduled, and we would plan on being back in London either Friday or Saturday to spend a week there before our flight home.
We don't want to drive. We like trains. We like eating. We don't like crowds. We love history. We travel slow. We like seeing maybe one or two "sights" a day and then spending the rest of our time wandering. My son is special needs and mobility-challenged, and he is in a stroller when we cover distances. So, no hiking or really bumpy terrain (we did bring him to Switzerland last year and hiked the wanderwegs; nearly killed the stroller, but we had a great time). The kids don't love art museums, but they are game for most everything else.
Where would you go in August? Some ideas I have: York, heading southwest (Avebury or Stonehenge, Tintagel, ponies in Dartmoor, is Cornwall too far? I think so), southern Wales, loop along the south coast (Portsmouth, Brighton, Dover), head up to Liverpool (or Chester?).