The V+ is a replacement for a standard STB.
As far as I know, it is likely you will only get a single V+ (although a few others apparently have more than one, depending on the area). The additional £5 per box you refer to is for a standard STB. Your maths are right - one of your boxes would be replaced by a V+ and you could retain your 2 additional STBs for a total of £20.
to the forums, BTW!