The national speed limit on a duel carriageway is 70mph, on a single carriageway it is 60mph.
Lets deal with the single carriageway fist, this generally involves country lane driving which can be a little daunting with the bends involved. What you need to keep in mind is the speed limit of 60mph is a MAXIMUM limit, it does not mean you should travel at 60mph, indeed if you tried to travel at the maximum limit you would no doubt find yourself 'close and personal with the local hedge'. You must travel at the speed that is SAFE for the road conditions, which means we have to consider several different factors. Firstly we must ensure we can get around the bends without going up the hedge or into the opposite lane, secondly we must consider other road users approaching us from behind (consider what would happen if you were travelling at 20mph in a 60mph zone, the car behind you could hit you at a relative speed of 40mph.
Now you may say “I am going fast enough at 20mph on this road”, unfortunately your test examiner might consider your 20mph to be dangerous and fail you for it, indeed even 30mph can still be considered dangerous of serious.
My advice is to try to get your speed up on the straight bits where you can see its safe to go a bit faster then as you approach a corner you need to slow down to take it safely, this way although you take the corners at a safe speed your overall average speed is kept high.