Zombie thread alert . . .
I do not know about your memory, but your sources have certainly not served you right. From the moment the first mineral trains ran in 1898, until the moment the last track was lifted in the 70s, the GCR’s London Extension was Up to London.
Both official GCR documentation, and contemporary newspaper reports, from the opening to mineral traffic in 1898, and passenger traffic in 1899, explicitly mention that up trains ran to London, and Down trains to Manchester, &c.
What you have heard is one of the big 3 “Great Central Myths”. It is a partner to:
1. “The London Extension was built to a particularly large loading gauge to accommodate future traffic from the Channel Tunnel”. It wasn’t, it was built to exactly the same gauge as the remainder of the GCR’s network, which was smaller than several other contemporary British lines. This myth appears to have started in the 1960s, and the first recorded instances are in literature produced to campaign against the closure. The actual, distinctly average, structure gauge of the LE is both a matter of public record, and preserved in surviving structures.
2. “Swithland Reservoir was drained to build the London Extension”. It wasn’t. When the Corporation of Leicester learned that the proposed new line would cross their proposed new reservoir, they went to considerable trouble to ensure that the construction of the former would not be permitted to disrupt the desperately needed supply of water from the latter. Consequently the viaducts and embankment across the reservoir were one of the first pieces of work on the contract, and were in fact built by the same contractors who were simultaneously constructing the reservoir, as a sub-contract to Henry Lovatt. The work was completed before the reservoir began to fill. This myth appears to have originated as a misunderstanding in the 1970s when an author had to write a caption for a S.W.A.Newton photograph which showed the empty reservoir with the newly completed viaduct, and wrongly guessed at the reason behind it.
If you will take my advice, you will treat with caution any book or publication which states any of the above myths as fact.