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I'm not going to defend the Highway Code, which in many respects is very poor, but note that the full rules for roundabouts are here:
http://www.highwaycodeuk.co.uk/roundabouts.html
Almost all of these rules also apply to cyclists. You've only quoted the (admittedly ridiculous, and it certainly shouldn't exist, as it is very bad advice) Rule 77.
There's a well-established ritual now that every few years, cycle campaigners have to fight a rearguard action against these rules being made even worse. The CTC is much to be commended for its excellent work on preventing this.
I just lifted this from the current HC. It must be written by idiots. I certainly wouldn't use their advice on Old Street roundabout ...
If you decide to ride round keeping to the left-hand lane you should
be aware that drivers may not easily see you
take extra care when cycling across exits. You may need to signal right to show you are not leaving the roundabout
watch out for vehicles crossing your path to leave or join the roundabout