I'm surprised a bit of chicken and gammon knocked you out of ketosis as it's mainly protein - you must be one of the 'sensitive' ones.
The good news is that because you were already in ketosis, drink plenty of water and you'll slip effortlessly back in. It's not like starting all over again
Here's a little analogy to think about.
Two people embark upon a journey to Scotland. One goes straight there .. A to B.
The other stops briefly a couple of times: once to have a coffee & loo break ... another to admire a particularly lovely view.
So there they are - both of them in Scotland. Which of them failed in their journey?
The answer is, of course, NEITHER of them failed. They both reached their destination albeit one took slightly longer.
The same applies to your weight loss journey. So you had a bit of chicken and gammon - maybe you've extended your journey by a day. In the big sheme of things, that isn't so bad is it? Point is - you WILL get to your goal.