You should check your VAT return submission and payment deadline in your HMRC online account. As a general rule, the due date to submit and pay VAT returns in the UK is the 7th day of the month following the reporting period.
More information about the applicable deadlines is available here.
Businesses on the Annual VAT accounting scheme have different deadlines. These deadlines are published by the tax authorities.
Also those businesses on the Payments on Account scheme have different deadlines. You can check them in the online information published by the authoriteis.
If the due date falls on a Sunday or bank holiday, the date is shifted to the previous working day.