A recent First-tier tribunal has upheld HMRC’s position that a trader cannot ask to be de-registered from VAT from an earlier date. If you find you may be better off de-registering from VAT (usually because some or all of your customers cannot reclaim VAT) and you are below the VAT de-registration threshold (currently £80,000) then you should contact HMRC as soon as possible. This can be done https://www.gov.uk/vat-registration/cancel-registration.
Note that if your turnover over the NEXT twelve months (forward looking) is likely to be less than the de-registration threshold, and you can convince HMRC of this, then you can de-register now. You may not have to wait until your turnover has actually been less than the threshold.