Hello from a fellow Marmite lover I have never taken it, but don't anticipate it to be a problem.But do declare it. At MCO they have always sent us through the red line at customs, but at EWR this year they didn't want to know about our chocolate and candy gifts and told us to walk through green. They just ask what food and as long as you tell them about it, you are usually good to go. Worst case they take it off of you, but at least if you tell them about it there will be no consequence to it. But if they catch you not declaring ...
Just thinking, another option if you are desperate and willing to pay is to look and see if there are online suppliers who import British goods (as you get the US ones here) and have it shipped from the US to your resort/villa
You'll be fine as it's vegetarian, but declare it anyway. I declared our Weetabix and tea bags last year, so they just sent us over to get our bags x-rayed, it took all of 5 minutes.
We had no problem finding it in the supermarkets over there. We have managed to find it on every trip, last time we bought it at Supertarget but pretty sure we have bought it in other supermarkets in the past..... Quite pricey though!!!
I've taken it - no problems -always been sent through green channel. I usually take the little sachet things. Me & DD can't go a fornight without iteither