wrote:

As a matter of interest, apart from the Routemasters - which are the
oldest buses currently running on regular routes now?

If you mean TfL tendered routes only, then I guess you're looking at the
earliest low- floor single deck Darts. For example, routes 370/373
(Romford-Grays) operate with P-reg Darts. 1996/97ish vintage.

The earliest low-floor double deck Tridents were on an S-reg plate, I think,
although they didn't arrive en masse until the T-reg era.

Chris