Roundabouts don't really need speed limit changes, because they're naturally self-limiting. Most roundabouts have the same limit as roads they're on, so there are lots of roundabouts that have 70 limits on, say, dual carriageways [1]. But that doesn't mean you drive on them at 70.
Speed limits aren't really a speed that it's safe to go at throughout the limited area (for example, a blind T junction is probably unsafe at 20mph, but can still be in a 50). It's a speed that it's not safe to exceed, even when the road conditions appear to suggest otherwise. On a roundabout, as at other junctions without priority, the road dictates the speed, not the signage. Exceeding the safe speed indicted by the road itself is still an offense of dangerous driving, even if below the posted limit.
[1]: For cars; HGVs and vans both have a lower limit on dual carriageways. Not that many van drivers appear to know that.
Speed limits aren't really a speed that it's safe to go at throughout the limited area (for example, a blind T junction is probably unsafe at 20mph, but can still be in a 50). It's a speed that it's not safe to exceed, even when the road conditions appear to suggest otherwise. On a roundabout, as at other junctions without priority, the road dictates the speed, not the signage. Exceeding the safe speed indicted by the road itself is still an offense of dangerous driving, even if below the posted limit.
[1]: For cars; HGVs and vans both have a lower limit on dual carriageways. Not that many van drivers appear to know that.