My experience is different - now you have me worried!!!
I find it only takes me 5 mins every 2 months to enter the totals into ROS VAT3 form and pay online and then 15mins to do the RTD at end of year? The admin is only slightly more than having to keep the accounts and submit tax return etc which is the bulk of the work. It certainly would never encourage me to earn below the threshold - even if its a few thousand over?
Good to understand what the onerous part / cost is as I find it cost neutral apart form my 1hr per year admin? Is it highly dependent on whether its VAT registered customers you are invoicing (as it is for me)?
As I said for me the hassle was moving to doing a full tax return more than VAT itself.