I think the ability to add notes to Direct Debits and Standing Orders from the Scheduled Payments section in the Payments tab would be a better implementation than just adding tags. Notes added in the Scheduled Payments section would then be added automatically to the associated transactions in the Home feed when the scheduled payment goes out.
I guess including the ability to add notes to Faster Payments transfers would have to be implemented first on iOS though.
NB: these notes would be separate to the reference sent out for Standing Orders.