We do it to avoid the dopes that chose to drive on the right side of the road, as you are well aware of ,there millions of life's saved in England because of it....Plus us and the Germans who we won't speak of invented cars....:)
The left-hand side was once common - I read somewhere it was actually Emperor Napoleon who changed it (well before motor vehicles), and once France started driving on the right it soon spread across the neighbouring nations.
The British Isles do not have a road connection to the Continent, so had no need to change.
Now though, if you look at a modern road atlas and realise just how densely supplied with roads we are, then consider that virtually every road and junction from the quietest rural lanes and housing-estate streets, to the motorways, are marked and sign-posted, you would realise it would be impossible both technically and financially to change. In fact the UK Government is spending a fortune of tax-payers money on installing traffic-management systems on some of the motorways; greatly increasing the number and complexity of the overhead signs all facing left-going traffic.
Besides money and practicality, changing sides is just not necessary!
Also you need consider all vehicles - taxis, buses and HGVs as well as private cars and light vans - are right-hand drive for left-lane roads with the kerb hence bus-stops etc., to the left.
Those few nations who did swap sides within the last 50 years or so, probably had fewer roads, lighter traffic and simpler road markings and signs.
====
As for that daft remark in the OP about measures, that is decades out-of-date.
The United Kingdom started going metric back in the late 1970s - 80s for almost all trade, including everyday shopping - though shops do use the non-"preferred" centimetre for clothes sizes. The very few legal exceptions include the Statute Mile and Yard for road distances and speeds; and the Imperial Pint for bottled milk and for serving keg and draught beverages in pubs, though there bottled drinks and dispensed spirits are in litre-divisions. Also in the flourishing trade supporting engineering-preservation and model-engineering, because these regularly work on very old designs and machines made to Imperial dimensions.
I believe the railways here still use Miles, Chains and Yards. (1 Chain = 22 yards.) They drive on the left, too, on double and quadruple-track lines; but the driving seat and controls are on the left (platform side) of the cab.
In doing so, not long after converting to decimal currency (in 1971), the UK joined all other nations in the world except the USA (which invented its own version of the ton and gallon) and one or two small, developing nations. Not only metric measures as envisaged by the French who invented them, but now it is the Systeme Internationale version which strips the lot down to a few fundamental units, their compounds and a narrow range of "Preferred" divisions and multiples.
Young, non-Americans now would probably paraphrase the OP to ask why "the confusing American system of measures?"!
When America will follow suit is an interesting question; but it is inevitable. American scientists and engineers do work in SI units especially in international projects; anything the USA imports now will almost certainly have been made to those units; and surely its own exports must have to conform to relevant standards all using SI units even if the items are still made in inches rather than millimetres.
And yes - those are the correct spellings (French).
'' I have encountered one odd exception though, from Europe. I once saw a programme from a recital celebrating completing a major overhaul of Oslo Cathedral organ. Now, Norway has been all-metric since I've no idea when; and the programme was written in Norwegian of course. Yet its description of the organ quoted the stops in fus (feet - the Imperial measure)! Well, naturally. We can't have Bach or Widor on "Diapason 4.877m", now, can we?
WelbyQuentin 