Rotherhithe - Canada Water on the ELL is now suburban rail. It’s regulated in exactly the same way as other parts of the Overground, as normal heavy rail. Yes it’s relatively high frequency, but it’s not a metro.
The Railways and Other Guided Transport Systems (Safety) Regulations 2006 Sect 2A give the ORR responsibility for determining and publishing an "Approved list of systems excluded from the scope of the mainline railway requirements" - the current (last updated in April 2026) version includes, under "Metros and other light rail systems", East London Railway from Highbury & Islington (only ELR exclusive tracks) to New Cross Gate, New Cross and Old Kent Road Junction.
So that part of the ELL is metro, and not mainline.
Railmiles gives Rotherhithe-Canada Water and Ty Glas-Birchgrove as 17 chains (=342 metres). While LU kilometrages give a figure of 0.32 km for Rotherhithe-Canada Water (slightly less), I have not seen a more precise figure for Ty Glas-Birchgrove. But as the East London is a metro (see above), it takes the title.
If you are going to count metros, then Canary Wharf to West India Quay on the DLR is only 199m, and has been declared Britain's shortest train journey - see
today's (23 May) diamondgeezer blog post (and click 'See Transcript')