There are always restrictions on travelling to zones of potential conflict. If you check the Foreign Office website you'll find directives there advising the individual. You can always check with your sqn/coy office as well.
You can always ignore the advice but if you get into bother then you'll be in hot water too.
I think you are refering to countries where special security regulations apply. All you need to do is inform your unit security officer that you are going they give you a quick interview before and after you go. Not sure what the current list is but China, Russia, Cuba and Egypt are definetly on the list but you can still go. Hope this helps!