My buns get leaf lettuce, parsley, and cilantro (I believe you call it corriander in the UK) every day. I will rotate a small amount of other things. For example, today I got some brussels sprouts, so every day this week they will get their normal salad, plus each bunny will get 1/4 of a brussels sprout. (1/2 if they're small)
Then next week they will get normal salad. Then the week after that they will get something different, maybe kale. Again, they will only get a small amount of the kale.
All my bunnies like to try new veggies (Amelia is picky about fruits but not veggies), but Nick has a very sensitive tummy, and will get squishy poos if his diet is wildly different than normal. I do like for them to eat different things, though, I think it's good for them to get different nutrients and they seem to like trying different things.
I am very conservative with the cabbage-y foods, I do not want to cause gas. In the summer, they will get larger amounts of the different things. Particular summer favorites: organic dandelion greens; carrot tops; mint leaves. Occasionally I will give them small bits of peeled cucumber. I also sometimes cut a baby carrot into fourths, and give each bunny two little pieces of that. Not every day, though.
As long as you stick to the "safe list," you are probably fine as long as you always give new foods in moderation.