No, that isn't allowed; but making fun of the colour of a white person's skin seems to be!
It doesn't refer only to white people. The gammons, are usually white, but not exclusively.
"Rosebifs' would be considered a pejorative term for English people, if you really wanted to be offended by it.
And you do seem easily offended, even offended by terms that aren't racially/ethnically/nationally/culturally or religion-wise offensive, such as 'gammon', although it is a political jibe, as is 'snowflake'.
Incidentally, make no mistake, I do not like, appreciate or in any way think that demonising people because of their political beliefs is acceptable.
Prejudice based on race/ethnicity/culture/religion or nationality is not, IMO, a prejudice based on a political belief. It is pure bigotry based on the person's ideology, that other ethnicities, nationalities, etc are in some way inferior or to blame for socio-economic problems.