Where to start! Expensive but brilliant options:
Hampton Court Palace (so much to see and do and children’s audio guide available),
HMS Victory and the Mary Rose,
Tower of London,
Windsor Castle,
Greenwich (park, Painted Hall, Cutty Sark, Queen’s House, maritime museum, Royal Observatory) then take the river boat back to central London on the Thames,
most National Trust and English Heritage properties inc Stonehenge and the many castles,
Gardens: Great Dixter, Hidcote, Wisley, Heligan, Sissinghurst (loads all over the country),
country parks eg Moors Valley in Hampshire,
beaches: Norfolk, Devon, Dorset (inc Purbeck), Cornwall, Northumberland, Pembrokeshire
Cities such as York, Bath, Oxford, Cambridge, Edinburgh also Stratford upon Avon,
steam trains eg Bluebell line,
theme parks eg Paulton’s Park for younger children and Alton Towers for older ones,
Museums: great museums in Liverpool, to say nothing of Natural History Museum, Science Museum, British Museum
theatres in the west end and around the country.
The world is your oyster!