We don't ruin it the politicians do
OK so in addition to what everyone else has said...
Nitrous oxide is still legal to my knowledge.
hyoscine and diphenhydramine are legal and potentiate some depressant drugs (but as always you gotta be very cautious, it's playing with fire).
Poppy seeds? If you have no tolerance they can be quite powerful (and so again it's worth being cautious).
By and large nothing good is OTC anymore since codeine was removed. As others have said there's pseudoephedrine which has a mild, shitty, stimulating effect. And you won't be able to get it very often though since it's tracked to your name. I've only ever gotten it when I've genuinely been sick.
As always though, stay safe man. Sorry to keep repeating myself but this is a very dangerous road to be on.