It seems obvious - the R50 was designed in Britain and so had its fuel filler on the left side, against the curb on our drive-on-the-left roads. For the second generation, Mini kept that but I guess by the third generation all those German designers working on the car said "Ziss is not right. Der filler should be on ze right." So it got moved.

It's all a bit unimportant since I haven't seen a filling station right on the curb for twenty years minimum. Left side is good in Europe since most cars have it on the right side and so there is often a vacant pump for left side users.

 

You can park either side of most gas pumps and use the filler on either side of the car. I do it all the time.

 

Oh, you're that guy!

 

Yes.

 

The Mini is so mini you can pull the hose around or over the top and make a connection

 

It would be a frosty Friday in July before I would run the gas pump hose over the top of my car. Any car.

 

The Minis roofs are so low you needn't worry about the hose rubbing the roof.
The hose originates higher than a Mini is tall.
In fact......
Minis are so short you can back up to the pump and fuel from either side.