The Theft Act 1968 (I hope I've titled it correctly) appears to specifically permit anyone to indulge in wild berry picking, wild flower picking and wild mushroom picking (provided it's not done for profit and providing the growing organisms are not uprooted).... but only in England and Wales..... unfortunately, convincing people that said Act is not a Scottish Instrument and does not apply here is often like banging your head of a brick wall. Even SNH seem to have adopted it, North of the border...
Fortunately the Land Reform (Scotland) Act 2003, for all the confusion and disruption it is causing, does have, as one of it's few sensible benefits, the advantage of clearly stipulating many areas to which access rights do not extend and therefore to areas where "mushroom pickers" et al have no right to be. Unfortunately, again, few people bother to learn just how many areas are off limits and even the "Access Code" does tend to "interpret" the law here... Heck... I've even had an access Ranger try arguing with me (quite erroneously) about a private road through my own farm buildings, which is most certainly NOT a public right of way and is therefore land to which access rights definitely do not extend, but now I digress.
As stated before, just show a bit of respect and courtesy to the landowner and ask nicely. I know it's not fashionable to do these days but, in my experience, it nearly always helps you get what you want and saves anyone from getting steamed up. There's one guy I always allow on to pick mushrooms in cropping fields here because he gives me a share of my own mushrooms in return (usually just Horse mushrooms and common Field mushrooms)...
....But he did once find some Morels that I had never noticed or known about before, though he'll be a very lucky boy indeed if he ever beats me to the Oyster Mushrooms, Chanterelles or young Puffballs.
No-one's ever asked me about looking for Antlers but I could point someone in the right direction for those too.