I'm going to do a terrible thing... and post without reading the thread all the way through! (I'll probably be the one with egg on my face... fried egg of course. fried in lard )
I come from a working class background and married a toff (I'm allowed to say toff, ok). I was silent for 2 years, deeply ashamed of my cultural roots, terrified I would be embarrassed in some obvious way ormake an outrageous social faux pas.
Your ILs are in fact low class re The ILs don't criticise me directly - more passive aggressive "oh don't you do this" and general raising of eyebrows . They sound a bit socially insecure/climbing to me. Unpleasant tbf.
Perhaps you could approach it that you have married into a different culture. This isn't far from the mark - imo class social norms are cultural poles apart a lot of the time. The problem is that if you were say, german, and you were living in an english family, you would not hide it (and people would allow for it) - but you aren't coming out with it so people don't know. You're probably not coming out with it because you are ashamed of it, ashamed of your background.
But hang on: to come from such a deprived background and make your way to where you are, with a successful marriage and career, is stunning. You know it is, come on. You have done remarkably well and have nothing to be ashamed of - on the contrary, you have a lot to be proud of.
I eventually broke out of my self-imposed silence when I realised that I was witnessing a great deal of ignorance in the family I had married into, and realised that ignorance does not belong solely to the working classes. I wouldn't go so far as to say I am 'proud' of my working class roots, because I am neither proud nor ashamed. It is as it is. However, I will, if necessary, make it clear when my social/class norms clash with others'. The middle classes don't have all the answers though they think they do .
After a time of being out and proud, I have learned to tone it down. Class snobbery runs deep ime and I can't count the number of MC people who can never forget where I come from, making a comment about it Every. Single. Time. we meet. Tiresome. They are not rude about it, just can't get over it. A bit like being black back in the day I should've thought ie some people find it hard to get used to.
time to face and own your roots OP. If it makes you feel any better, I have a friend who is absurdly posh and hid it as though his life depended on it. It's not only those of us from the bottom of the pile who can be ashamed of our class roots.