It is not uncommon to see some payments stuck in submitted for some time before they are moved to executed by the bank. This can happen for several reasons, which are all dependant on the bank, but the most usual are for AML checks.
Some payments can take up to the end of the following working day. In this Faster Payments page, it states that
As a minimum, all financial institutions in the UK must abide by the Payment Services Directive (PSD) - this states that any payments made by mobile, internet or phone banking (including standing orders) must arrive by the end of the following business day at the latest.