In the past forty odd years, I have restored probably an even dozen vintage Brits of one species or other. Many with ignored and neglected leather.
I can second the reccomendation for Lexol. However, I can also add personal nods to "Treat"...the Zymol leather conditioner. It is a collagen formula which, used properly, does a very good job in preserving and rejuvenating tired leather as well as new. Their cleaner is so-so, it works well but nothing to write to Gerty about.
In a pinch, Tanner's Preserve is a serviceable product as well. It is cheap and readily available. But I would absolutely use one of the others if at all possible.
If you want the cheapest, yet effective, of all candidates....there is always Mink Oil. It is a reliable standyby that has been around for decades and which is available at just about any grocers' in town for a couple dollars a tin.
As with any treatment/conditioner, always try a new formulation on an area out of sight in case it might affect the colour/textureof the leather. However, I can say that any and all of the above have never caused me such problems in any situation.
I would also agree with Don NOT to bother with the combination formulas, no matter whose you go with. Get a cleaner and use it each time prior to conditioning.
I don't own a detailing shop and I'm not selling anything. Merely comments based on personal experience.
....sT