Am I understanding this correctly? Certain sites are open only July 1 -Aug 30? I've never heard this, but while checking sites (Althorp Park, Parliament, parts of Buckingham Palace)found some open during a late summer window...

Yes, as everywhere in the world, some places are open for only a small part of the year: dates vary by location. That's obviously not true of places that have no other purpose but to be shown to visitors.

Buckingham Palace has paid-for tours during summer only. Any other time of the year, you have to be invited to a function to see the inside, though part of the art collection is on display in the Mews year-round.

Parliament offers tours to people who don't live in Britain only during its summer recess. The rest of the year, residents need to contact their MP (MPs are required to work for ALL residents in teir constituency, and not just UK citizens) who'll arrange a tour. Non-residents, outside the recess, can come and watch debates, or conduct business: there just aren't any routine organised tours they can go on.

The three sites you list are special cases.

B'ham Palace is only open in Aug/Sep since the Queen is out of town at that time every year (mostly up at Balmoral)

Parliament has not allowed non-UK residents to tour except for during the summer recess for several years (and for a while - not at all)

And Althrop is the family home of Earl Spencer and is only open for a short time in the summer mainly to see the island where Princes Diana is buried and tour the gift shop.

None of these are typical "tourist attractions". You won't find much else closed during the summer.

that last should have read >>You won't find much else w/ limited opening times in the summer<<

Ok. Now I've got it! Thank you both for taking the time to share these detailed responses.

It seems every week or so DH and I realize there's yet one more thing we'd like to see while in England! We've even broached the subject of skipping France (Paris, Versailles, Normandy) this time and simply focusing on England (London, the Cotswolds or Peak District), for our 12-14 day visit June 09.

We'll see... Thanks again!