Embassies in London currently issuing visas (in most cases limited categories):

Afghanistan, Angola, Belarus, Cambodia, Cameroon, China, Congo Brazzaville, Cyprus, Gabon, Ghana, India, Ireland, Kazakhstan, Kenya, Libya, Malaysia, Nigeria, Russia, UAE, Saudi Arabia, South Korea, Taiwan, Thailand, USA and some Schengen countries (including Denmark, France, Germany, Greece, Italy, Netherlands, Poland, Portugal, Spain).