Is the UK the only country which drives on the left side of the road?

you don't have to look far to get your answer. Ireland, isle of man and the channel islands. I presume you already knew the answer to your own question. Why did you ask it? Do I get best answer now considering it was all a bit pointless anyway?

NO, it is not. Don't forget the Commonwealth countries! India, Pakistan, New Zealand, Australia, and Ireland, Malta, Falklands, Hong Kong, Japan and probably a few more. (Samoa, Fiji?) Obviously for the majority of European visitors, it is the "wrong" side, and for most right handed people, operating a manual shift with the left hand seems awkward. The reason for driving on the left hand side of the road goes right back to the days of horse transport, when a solo rider, seeing another horse and rider approaching, had to be prepared to defend himself against a possible robber. The practice was to move towards the left hand side of the track and draw a sword with the right hand (as most people were right handed). The other rider would have the same concerns about his safety, and would do the same. The two riders would then pass each other, sword arm to sword arm, thus, well placed to defend themselves against any threat. When cars came into use (contemporaneous with horse traffic) the car drivers simply followed the same practice, although there was probably, no longer a need to carry a sword.

No they are not. Here in Australia also New Zealand and most of the Pacific islands we all drive on the left. I also think Singapore and Hongkong and possibly most Asian countries are left hand drives also

It's not the "wrong side of the road" in Japan either!

As well as the places listed above, many other ex british colonies do. Parts of Africa, India, etc.

It's not the wrong side of the road,

Thailand do Well mostly, but when I saw everyone overtaking in any lane, as I was driving along, I asked my Thai wife which was the fast ( overtaking lane ) she replied " All of them "

no

left was the natural side to ride on in the days of horses when passing another horse, for signaling to a passing rider or defending oneself with a sword. Napoleon ordered that the right hand side be used, causing continentals to drive on the wrong side. Driving on the right gives no advantage and gives the disadvantages that signalling by hand would have to be done with the left hand.