I dont think if the information is elsewhere on this forum, but there are threads on Euroga.org and UKGA.com where there is a freedom of information document published on this exact topic.
From what I recall, the essence of the information is that London City controllers will clear you into their zone as it is not their job to police you based on your aircraft and requirements to be able to glide clear, but thats not to say you should go in. The Thames is apparently not defined as a suitable land clear destination, but it also seems that one would be unlikely to be prosecuted unless you actually endangered yourself or someone/something else. There was a case where someone was in the zone and ran out of fuel and just managed to land at the airport, and that annoyed some people was the subject of the debate within the FOI request.
Its up to you whether to chance it, and it's true you will get some good photos. I dont think you can get a definite yes/no on this.