@Resurgam @Caeseji @Tophat1900 @Jim Lahey @DavidGrahamJones really great strategies for dealing with socia lunch and dinner situations.
What I have also noticed is that almost everyone is totally unaware of what I am eating (or not eating) and much more interested in choosing their own food. Once I realised that - social situations were much less stressful and I could just relax and enjoy them while feeling extremely virtuous. A small bar of 85% chocolate is my favourite emergency food if there really wasn't anything to eat.
I will also take a little flask with some cream for my coffee. This doesn't work if you have done a 5 mile walk before stopping to eat. The cream has been churned into a solid block that does not mix easily with my coffee.