Others have advised of grounds on which you can challenge the penalty fare. But it seems to me that you also need to be aware of the railway byelaws which you can find at
https://assets.publishing.service.gov.uk/media/5a79c14b40f0b66d161ade8c/railway-byelaws.pdf. It seems to me that byelaw 6(8) is relevant (and also byelaw 24(1)):
It's against the rules to travel in first class without a first class ticket: this has been dealt with by the penalty fare. But it's also against the rules to 'wilfully interfere with the comfort or convenience of any person on the railway'. We know that railway staff dealt with you in an otherwise empty first class compartment - but they are still persons. And since in your original post you tell us
you seem to accept that your food could be annoying, and that this would be a problem for other people - so you would be wilfully interfering with their comfort.
So if part of your appeal is going to be the consideration you were showing for other passengers, bear in mind that that will mean admitting a byelaw offence which the railway could prosecute you for.
We try hard not to be judgemental here, but it seems to me that whatever your motivation was, you've been caught bang to rights breaking the rules by travelling in first class without a first class ticket. If you don't appeal, you'll have to pay the penalty fare: if you do appeal you will be admitting to an offence that you could be fined for. Given that there's very little doubt that you were breaking the rules, my suggestion would be to pay the penalty fare within the 21 days allowed which reduces the penalty from £100 to £50, along with the first class fare that you were also charged.