I find it amazing that First Capital Connect is slammed so much for not being customer friendly because we do not allow standard class tickets in First Class and they only have small first class compartments. EMT had 3 carriages of First Class and they did not declassify!
Last winter, some of my use of EMT's services included a heavily delayed journey from St Pancras to Derby. Due to a lack of direct trains to Derby (yep, things were that bad) I had to change at Leicester. The available Meridian for the final leg of the journey was so full that any and all passengers were forced into First Class accommodation, and then into the seats, because of the sheer volume of standing passengers piling onto the train. No formal declassification was in use, although the guard was urging passengers to move right down inside all the available coaches.
You usually find that a huge crowd that's waited on a platform for a good while in sub-zero temperatures will go on to occupy whatever available space there is on whatever train turns up. I have seen similar on Southern services a couple of times, but the size, positioning and separation of First Class is somewhat more conducive to these types of problems.
Outside of exceptional circumstances like these, however, I would strongly argue in favour of penalties for those who have made the choice to sit in First Class without a valid ticket. If you want an almost-guaranteed seat, buy the correct ticket. I can't see a fuss being made on a genuinely crush-loaded train during major disruption, but anything else and you should seek permission or face the legally-enforceable consequences.