Quite a few of the Travel.SE questions on UK train travel suggest getting a railcard to reduce the cost of travel. While not all train tickets can be discounted this way (some railcards are area restricted, others only start part way through the day, others need a certain number of people to travel together), and you have to buy the railcard, for someone doing a fair bit of UK train travel they can deliver savings.
Quite a few of the railcards have restrictions, such as only being available to older people, or to young people + those in certain FTE. Some are restricted by where you live, eg this one.
Assuming you're of the right age to buy a given railcard, living in the right area (if appropriate), what about nationality? Are there any restrictions on any of the railcards which would prevent expats living in the UK from buying or using them?