Most seeds and plants that are things designed for humans to eat are VAT free. That includes seed potatoes and onion sets, as well as fruit trees, soft fruit and small herb plants.
I may be wrong on this as it is a while since I worked at a garden centre, but I think a 2L pot is the limit for VAT free on herbs. Anything over that size is classed as an ornamental plant and there is VAT on all ornamental plants, seeds and bulbs.
I don’t think it makes any difference if the veg seed pack is pictorial or plain as regards VAT.
Fertilisers and additives like lime are VAT free, but I think you are right about bagged compost having VAT included in the price.