Oldest mainline certified loco?

[Bittern]

Upon seeing an image of Caledonian Railway no. 123 and wishing to see her restored to running condition, I got thinking about her age - 125 years old - and if it would be an issue if she were to return to steam and get mainline certified. Upon this, I got thinking about the age of certified locos and which of them is the oldest.

Any ideas? I would think Scotsman's a contender being born in 1923, but is she certified at the moment as she's about to come out of an overhaul (this year!!)?

 

[33056]

In the UK as a whole, the RPSI's no. 186 , built in 1879 is almost certainly the winner.

 

[GM078]

In the UK as a whole, the RPSI's no. 186 , built in 1879 is almost certainly the winner.

Indeed, are there any 19th century locos registered on the mainline in England/Scotland/Wales? Some might argue as that 186 is in rebuilt condition (1930s) she would not be 100% the same design as it was when built (its sister 184 is closer to this condition). But at the same time she still dates from 1879 even if changed slightly, so probably is valid as the oldest loco (or even rail vehicle in general) registered for mainline use in the UK or Ireland.