I've just had the sleeves of an off-the-shelf suit jacket shortened ("I'm not a gorilla") at the mobile phone repair/dry cleaner/clothing alterations/massage and beauty treatment parlour on The Cut near the Young Vic.
They offered me a choice of shortening at the cuff or rebuilding the shoulders (the more expensive choice) and did an excellent job of the latter. I've used them in the past to let out trousers when they've shrunk in the wardrobe, and they did a good job then.
I can recommend them and have no connection other than being a satisfied customer.
I've used the same people who have been incredibly efficient - and patient - about rescuing and patching an ancient jacket, speedy shortening of son's new trousers, and emergency crotch repairs on the trousers of husband's only suit, highly recommend