Left-handed traffic is rather simple: you have to look at right-handed traffic through the mirror. The cars drive on the left side, a roundabout works clockwise, and you overtake on the right side. So much for the theory. In reality things are a bit more complicated.
If you come from a country with right-handed traffic, and you travel to Great Britain or Ireland, you will see the effects of the left-handed traffic immediately.You don't have to drive, you just need to cross a road: the old rule to look left first, then right, does not work anymore, it is the other way round. And if you try the short version of looking left and step on the road, it might end badly.
In short: the long trained habits and instincts don't work anymore. That applies for driving and walking, and sometimes you have to be focussed not to get yourself into dangerous situations. No wonder the pavements in London or Dublin are marked with "look left" or "look right".
Driving is even a bit more challenging. I know a few people who would never dare to come to Great Britain with a car, just because of the left-handed traffic.That is a shame, since it is not that difficult after all.
You should prepare yourself mentally for the situation. It is important to get yourself aquaintet with the effects of the inverted driving: left turn means directly left, no need to look at the cars that come towards you. But turning right means you do need to give way to the traffic on the other side, and you need to be aware that you turn into the left lane.
If you think it through before, it will be easier when you actually do it. You need to be aware that you cannot just drive off and trust your habits. But on the other side there is no need to be afraid, since it is feasible, in running traffic it is easy to see where you need to go. It is only important to pay full attention while driving.
In the end the driving on the left is a matter of training. If you have driven for a few days, you will quickly get into the inverted traffic world. The best to start are the motorways and double-lane roads, these are rather uncomplicated and you quickly know how to use them. It is also a bit easier when the traffic is a bit heavy, since then you just need to follow the other cars.
One way to keep yourself aware of the left-handed traffic is to use a hire-car. Then the driver sits on the right and has to change gears with the left hand, that way you are always reminded of the different setting. The other advantage is that you can see better ahead, if you are stuck behing a lorry or a tractor. With a continental car you sit on the wrong side and you need a long curve to see and overtake.
Also, if you enter a car park or a toll station, you sit on the right side. I have sunk coins into the car door while trying to pay from the "passenger" site. One time I just got out and walked around the car to hand over the money, what resulted in puzzled (and slightly amused) looks.
On the other hand, if you take your own car it has the advantage that you don't have to get used to a new car, and you can concentrate on the driving.
In the end you have to decide for yourself, which variation is the best. It also depends on the travel plans, like if you are traveling with several people or a family, then taking your own car is probably cheaper than train or plane tickets and hiring.
By the way: British cars are not inverted but just shifted. That means the accelerator is right, clutch is left, the ignition is right and signal lever left. It is only the seat and the wheel that is right, and therefore the shiftstick is left. I'm telling that here because people ask that a lot.
Traffic rules are rather similar to the continental ones, and also the signs are more or less the same. Despite that, there are some slight but important differences, apart from the left-handedness.
Different from German rules it is not only not prohibited to signal before entering the roundabout, it is even requested. That makes sense, since the cars that follow know where you want to go and where you are heading in the roundabout.
Big roundabouts have several lanes to enter. It is important to get into the proper lane depending on where you want to go. This is signed in most cases, but generally you enter left when you want to turn left, and on the right lane when you want to turn right. For straight ahead you can use both lanes in a double lane road.
Generally the traffic in the roundabout has the right of way. That is nothing new, but there is another effect by this: sometimes the roundabouts consist only of a painted white circle on the road. In this case the traffic from the right has the right of way, meaning "right before left". But that is the only situation where this continental rule applies!
In a big roundabout you will notice that the lanes are not going parallel to the rim, but leading like a spiral to the outer side. This was made to lead the traffic to the exits which are normally marked in the lanes (e.g. "M 1 South"). If you stay in this lane you will be automatically directed to the right exit. In other cases you change to the left lane before the exit and signal.
Generally the straight-on roads have the right of way, meaning that on a T-junction the road that ends has to give way. If you have to give way, there is normally a double broken line painted on the end of the road to mark this. Mostly there is also a simplified "give way" sign painted on, an upside-down white triangle. These signs have to be observed, since there may be no other signs next to the road.
In Great Britain and Ireland the speed is measured in Miles per hour (Mph) instead of Kilometers per hour (km/h). Therefore you should know roughly how to convert them (1 Mile = 1.6 Km)
10 Mph = 16 Km/h
20 Mph = 32 Km/h
30 Mph = 48 Km/h
40 Mph = 64 Km/h
50 Mph = 80 Km/h
60 Mph = 96 Km/h
70 Mph = 112 Km/h
80 Mph = 128 Km/h
If not given otherwise, the limit in a town or village is 30 Mph, meaning 48 Km/h. In town does not mean between the town signs, but in a built-up area with street lighting.
Outside on single lane roads the speed limit is 60 Mph (96 Km/h), and 70 Mph (112 Km/h) on double lane roads or Motorways. These limits are for normal cars without trailers, or motorcycles.
If you drive on a British Motorway, you will see that not many people stick to the speed limits. This is in some way surprising, since a speeding ticket has to be reported to the car insurance, and with the second one the isurance premium goes up, the same with other offences. Something like that is unthinkable in Germany (yet?).