As VAT registered businesses you will be able to view the VAT you owe to, or can claim from, HMRC. Initially this only supports the quarterly accrual accounting scheme. If you are not on this scheme you will still be able to know your VAT deadlines and the VAT you have paid or claimed back in previously submitted VAT returns through the Making Tax Digital service.
For customers who have given access to their data to their accountant, their accountant will be able to submit their VAT return (Making Tax Digital) straight through the Accountant Hub. This is ongoing work and you can follow progress here: https://countingup.canny.io/accountant-hub-ideas/p/submit-vat-return-making-tax-digital