I'm sorry, but that is what tends to really get the goat! That very attitude, that 'there are no seats in standard so I should be able to sit in First'. No, no and no!
First is less full because it costs more. If people wish to enjoy that extra benefit, there is a requirement to pay for it accordingly. I have in the past asked a number of these people to explain to me how they feel the First Class product works on that basis; that we will charge for it and then let everyone else sit there anyway. Strangely enough, I'm yet to get a decent answer. I've also offered people the opportunity of staying in First, if they can stand up and persuade the rest of the carriage that they should be allowed to travel there for nothing whilst everybody else has paid the fare due. Oddly, that doesn't seem a popular option either.
Unless a train is genuinely crush loaded, which they really aren't all that often (i.e. "I have walked all the way through the train and there aren't any seats...." - but clearly there is plenty of room to stand then!) then I'm afraid it's a case of tolerate standing room only, just like everybody else is doing, or pay the fare. Grrrr!