My family (me, husband, 10 year old) will be flying into Heathrow a few days after Christmas and staying through the first week of January. This is our first time in the UK. We would like to leave London for a few days over the new year holiday to avoid crazy crowds and then come back to London to finish out the trip. Any recommendations of where to go for 2-3 days during that time? We do not plan to rent a car, so reachable by train is ideal. Also we are planning day trips from London to see Stonehenge, Windsor Castle, and the WB Harry Potter Studio Tour, so it wouldn't need to involve those items. I was considering North Wales, but I'm not sure it will be so easy for us to reach by train over a holiday, and I'm also concerned there may not be enough to do that time of year. Any thoughts?
How about York? It is an easy 2-hour journey by direct train (no changes).
Use this website to check on train journeys:
Instead of trying to do a day trip to Stonehenge, you could consider spending a few nights in Salisbury for your getaway time. There is a local bus line that runs a shuttle route out to Stonehenge and Old Sarum. The current schedule page only shows thru mid-Oct so you'd want to check to see what their times are in winter and thru the holidays.
http://www.thestonehengetour.info/timetable
There is also Salsibury Cathedral (has one of the Magna Carta), the Salisbury Museum and an interesting town around them.
I will tell you I have NOT been here in winter.
https://www.visitwiltshire.co.uk/salisbury/whats-on
You may also want to go to www.sunrisesunset.com to check your daylight times. You'll have short winter days.
The only places where there will be "crazy crowds" in London on New Year's Eve will be Trafalgar Square, and the banks of the river. Stay away from those and you'll be fine.