It’s a freakin’ BUG! Step on it!
In CA, AZ, NM, or TX, that's what I'd do, but on the London Underground, I'd call the authorities. It's not that I would feel inadequate to the task of squishing an arachnid on the other side of the Pond, but I would be concerned that no one would bother with identification for the resulting goo. It's worth at least attempting to identify the species and its source, and thinking about whether the West's enemies might have decided to start releasing such wildlife in parks and public transit.