Sorry to burst your bubble but there are a couple of major issues:
1. You aren't going to find 4 UC seats to Disney over half term. Unicorns are more common. It doesn't matter if you book a year in advance.
2. You cannot upgrade the cheapest K class PE ticket, so you are going to pay substantially more for H or you need way more miles to upgrade from Economy Classic
This post is a bit old, but I tried to analyse the value of upgrading VS flights...
https://insideflyer.co.uk/2017/02/up...rgin-atlantic/
It's hard to judge Avios availability vs. Virgin Atlantic. That said, BA do guarantee 2 Club World award seats on every flight, so you can potentially grab those as soon as they go on sale.