Royal Mail has advised that not all mail was processed to schedule yesterday at the following Mail Centres:

Carlisle Mail Centre: Some mail posted in the CA and DG postcode areas yesterday for delivery today in other parts of the UK.
Croydon Mail Centre: Some mail posted in the BR, CR, SE and SM postcode areas yesterday for delivery today in other parts of the UK.
London Central Mail Centre: Some mail posted in the EC, N, W1 and WC​ postcode areas yesterday for delivery today in other parts of the UK.
South Midlands Mail Centre: Some mail posted in the CV, LE, MK and NN postcode areas yesterday for delivery today in other parts of the UK.

Customers should expect delays in these areas.