Where to buy a suit that's comfortable when it's warm, in London or San Francisco?

The suits from http://www.ctshirts.co.uk/p/gbpdefault/r/Web-Department/Men's-Suits-_and-Separates/productlisting.aspx?ppp=12&sortBy=Relevance&page=1&back=False&canned=&browse=Y are supposed to be generally fine and they seem to be having a half price sale on cotton summer suits at the moment. Austin Reed is always worth a look for a suit too. Linen is ok as long as you don't need to be too formal. Also, I'm sad to say, don't go for seersucker in England. Unfortunately you will look a fool.

Get one made! Raja Daswani - famed tailor from Hong Kong now services US, UK, Australia, and some other places. According to the FAQ - "Suits range from US $350 to US$5,000, also depending on the material." So your price range fits in there. Anyway, it's an idea - http://www.raja-fashions.com/. The reason I suggested it is Raja Daswani is based in Hong Kong, which is a pretty hot and muggy place most of the year, so he'd have a fair bit of experience in making suits that suit warm weather / climates.

One of the HK/Shanghai roving tailors (Raja, Mohan, etc.) would probably be able to do you a wool/linen or cotton/linen blend. (Linen for breathability, the other to limit creasing.) I've heard mixed things about Raja, but you have to assume, as awfurby says, that SE Asian tailors know how to make warm weather suits. Not everyone can pull off the white/cream suit, but poplin or http://www.haspel.com/seersucker.cfm give you some room to fit your style.

Seersucker suits are awesome.

I'm going to second phuuniemee and tell you to go seersucker. It'll make you look like a southern gentleman, and it'll really make a statement. Oh, and look, http://www.josbank.com/IWProductSearch.process?Query=seersucker&SearchPage=1&RFCategory=Fabric&RFValue=Seersucker&SortBy=BSL is having a sale on them. Time to restock my selection as well. Seriously, go seersucker, never regret.

Bonus: the Rohan jackets usually come with a zillion hidden pockets.

You'll get a pretty nice suit with $800 in San Francisco. A http://www.yelp.com/topic/san-francisco-where-to-buy-a-great-suit http://www.yelp.com/topic/san-francisco-places-to-buy-a-styling-suiton this. The Men's Warehouse is a local chain that sells semi-discounted suits. Your budget will not be a problem there. Macy's is about the same or maybe one step up, and again, you'll be able to buy most of what they sell for $800 or less. Nordstrom is a step up from there, but l'm guessing you can still get a nice suit within your budget. Nordstrom also has a reputation for good service, though my experience is that they can be snotty. I don't live in SF, so I can't give you an opinion on the smaller retailers, but I'd definitely check with them as well.

The only black suit I've ever worn that felt cool in the summer was Armani. But that was ten years ago, and outside your price range even back then. I'd go with a well-cut off-the-peg linen affair or similar. I've had one of http://www.travellinglight.co.uk/index.php?option=com_oscommerce&osMod=index&Itemid=27&cPath=2_11_59&osCsid=b4203e8b33922bfa51efe8f69254aee for six or seven years, and it does what it says on the label: it doesn't crease in the case and all but the most major dirt and stains will wipe off it. Plus they're having a sale right now.