On the aside of etiquette in legal London, I was within recent memory pulled aside for not wearing a waistcoat. I think fashion has, however, accepted covert coats just about anywhere, even in the Inns of Court. Formal hats such as fedoras are making something of a comeback this year.
Thank you all for the continuing cloth recommendations. I've just stopped to look at the Thornproof book in a local tailor's. My first impression was that I liked how tightly woven the cloth was, and instantly preferred this bunch to any harris tweed. I looked at some Bill's donegal tweeds, but I wasn't as taken. I might well pick a cloth from the Thornproof bunch, unless I can find a really small patterned black/white herringbone (with a little blue interest, that would be perfect!). The herringbone tweeds I have seen are too bold with too wide a repeat.
From the Thornproof:
62263 is a lovely mid-grey, but the colour is possibly too plain.
62270 (grey), and 62271 (blue) are top of the list at the moment in the 18oz. All of these are the donegal-like tweeds.
On weight, I was originally thinking of something a little lighter, around 16oz. The only heavyweight suit I have is a dress suit in 18/19oz P&B barrathea, and that does wear quite warm, although it is very structured.
The 62253 is a lovely cloth (similar to 62270?), and is 12/13 oz. I've had worsteds in that weight before, and decided to opt for 14-16oz as standard city suiting. Do any of you have any experience of the 12/13 oz thornproof? I think it might be a little on the flimsy side.
My gut feeling is this is a balance between wearability for more of the year and utility. My usual mode of transport in town is almost indistinguishable from this
https://uk.pinterest.com/pin/419749627744929867/ (save that mine acquired blackout paint over the chrome), and the 18oz fabric might well survive cycling much better, despite the 13oz stuff being called "thornproof".