SEGA Mega Drive Controller not available yet in the UK.
The N64 Switch controller which you can order through the My Nintendo Store (only if you have a Nintendo Switch Online subscription) is currently sold out. The American website list them as being available again in 2022.
The controller for the SEGA Mega Drive is not available yet in the UK, but has released in the US. Both of these were released to coincide with the Nintendo Switch Online + Expansion Pack, which features games from the N64 and SEGA Mega Drive.