If you plan on traveling around England, Wales, and Northern Ireland and plan on visiting a lot of historical sights, then I would recommend joining the National Trust or English Heritage. Within two or three visits the cost of the annual membership is well and truly paid for and you can visit places you might otherwise not have considered.
Which one you choose will depend on what you want to see and how long you will be in the United Kingdom traveling around. If you are trying to decide which one to choose, have a look at my reviews of the National Trust and English Heritage.
Visiting the United Kingdom with Kids
The United Kingdom has an amazing amount of activities for families and is very family-friendly. And, despite the popular disparagement of local cuisine, I found that I really enjoyed British food. For a long time, I had incorrectly assumed that the United Kingdom was all very similar. I was certainly not aware that Welsh is still very much spoken throughout Wales or how different southern England was to the north. I loved every minute of our three-month journey throughout England, Scotland, Wales, and Northern Ireland.