With regards to Virgin the naming of aircraft has 2 reasons.
One is PR. I think a few of his have names that are used to "stick it" to rival airlines, The Spirit of Sir Freddie being one of them. Freddie Laker was put out of business by several other rival airlines and he gave Richard Branson a lot of advice on the likelihood of a dirty tricks campaign by rival British Airways. Many airlines tried to stop Virgin from even flying in the first place and thus the name was a defiant shout to those airlines (naming no names here but read Richards autobiography for full juicy details).
The second reason is for safety and is the reason why trains run by Virgin are all named too. When you are working on a train or plane its extremely hard to keep track of numbers, but very easy to keep track of names... "Oh yes, that repair was done on the Spirit of Sir Freddy last week..." is way better than "oh yeah, I did fix it last week, but I'd have to go look up the plane... not sure which one it was.... hold on let me call you back"