When you see the diagonal black strip sign below it means that the National Speed limits apply depending on the road type. Remember if you see street lights assume it is 30 mph unless you see a different sign. I am not sure if any other country has Roundabouts as a means of joining roads. I guess they can be a bit worrying to negotiate but the rules are a lot more complex to describe than to do. When you first see a busy roundabout you may find it a bit chaotic, but they are a fast means of integrating streams of vehicles.
Roundabouts come in all sizes.
They usually have 3 or 4 roads radiating from them, but this can be many more for the bigger roundabouts. Apart from mini roundabouts that are usually found in towns, they usually are well signed, both on the approach and at the entrance and exit junctions. This enables you to find which exit you require so that you leave the roundabout on the correct road. So as you approach look at the signs and count which exit you want. That is if your required road is the third exit then when on the roundabout you have to pass 2 exits before you leave the roundabout on the third.