I did the same thing once 😄 I was used to the string wrap thing they normally have and didn't notice the plastic one. Fortunately like you I found it before it went in the oven 😄Didn't kill anyone or make us ill.
If I buy a gammon joint from Sainsbury’s it usually has the plastic collar. I leave that on when cooling in the slow cooker and just cut it off afterwards. Absolutely fine and no ill effects after many years. Enjoy.
Just taken my rolled ham joint out of the slow cooker. It's been simmering in coke while were on our boxing day walk. I didn't realise it was actually wrapped round with a plastic strap! I've covered it in treacle and sugar etc anyway and put it to roast for half an hour.
Will it be OK or do I confess I might have poisoned my family??