Appropriate casual dress in London, late August
 

Although I suspect slacks are fine everywhere in London, I was hopping I could bring my blue jean slacks since they are more comfortable. I have some trepidations: deplaning at Heathrow are going through immigration, possibly the officials will be 'stricter', off course not to use at Buckingham Palace, etc, would they be acceptable in pubs? etc.

Jeans are ubiquitous and generally acceptable. But they are hot to wear and take an age to dry if you are caught in the rain. Wear what you feel most comfrotable in. London sees all sorts of clothing and no-one blinks an eye. Jeans will not be a problem to the order Force agent who checks your passport, assuming you even see one and don'use a machine.

Before you decide what to wear (which can be just about whatever you want) wait until a couple of days before you fly out. While the weather forecasts are very changeable -- it will at least give you an idea. I just returned from a week in London when it was high 70's to about 80F (which is very warm for London) and would have been too hot for me to be comfortable in jeans. But as soon as I left it became cool and rainy -- and jeans would have been totally fine. Then part of next week is supposed to be BLAZING hot and jeans would be gawd awful.

You get the idea. I never plan a London/UK wardrobe until shortly before the trip.

I'm not quite sure what "blue jean slacks are," but blue jeans are certainly fine to wear just about anywhere in London. As noted, though, they are heavy and cumbersome and take forever to dry, so not exactly the most practical choice. And do check the weather before you leave, for what that's worth.

It's not in an immigration officer's job profile to care one whit about what anyone is wearing when passing through immigration control. You can go through in a pink lamé tutu, and as long as your papers are in order and you don't have anything unusual strapped to your body you'll be fine.

Wear whatever you want. Jeans are fine to wear at Buckingham Palace unless you're invited to a garden party by the Queen.
I really cannot think of anywhere in London where jeans would be inappropriate.

If the OP is calling them "blue jean slacks" (and assuming this is a serious post) then I think they are experienced enough in life to understand the downsides of wet jeans. EVERYONE wears jeans EVERYWHERE in London. If you're most comfortable in jeans, bring them and wear them.

Unlike just 10-15 years ago, everyone now also wears shorts everywhere in the summer when it's warm. No dress codes at all. On the continent they care more about appearance but in Britain, anything goes. Lots of slobs. Don't worry about it.

I wouldn't put it past some clown in the Home Office to suggest that turning up in more formal attire than that suggests an intention to go to a job interview or to get married and therefore to stay!

Ha! Good point.

You only need to dress up for Buckingham Palace if you're going to a garden party (or otherwise meeting the Queen). Tourists can wear whatever they like.