I've been looking at this today for my GTV. The website is stating the tax will start from 1st March if I buy now.
Had a read through the small print on a V10, which you need to fill in if taxing while SORN at a post office.
"Section B. WHEN TO APPLY
IF YOUR VEHICLE IS NOT TAXED OR HAS A SORN
If you apply in the last two working days of a month, the tax disc can start on the first day of the next month. If you apply before then, your tax disc will be backdated to the start of the current month. (Saturday is classed as a working day at Post Office® branches.)"
Looks like we have to apply from Thursday at the earliest for the tax to start 1st April. Don't know if the same applies when taxing online, I'm going to try it first thing Thursday morning.