baremadness "in the uk...marriage is not religious"
Completely incorrect. Religious marriage exists worldwide, wherever there are religions that engage in religious marriage ceremonies. It has nothing to do with countries.
To say someone cannot have a Catholic, Methodist, Hindu or Sikh wedding because they live in the UK is crazy. They CAN have as many religious marriages as they want- they are just not recognised by the State.
What you mean is that in the UK civil marriage exists, which is the one referred to in legislation and official paperwork. This is secular. It is a piece of man-made contract law with no necessary relation to Christian or other religious doctrine. As opposed to countries where the church and state are less separated and the only type of legally-recognised marriage is also a religious one.
The issue arises because many people combine a State (secular) and church (religious) wedding, wanting to be seen as a married couple both in the eyes of the law [to get the benefits and protections that come with legal spousal status] and in the eyes of God [to get spiritual blessing and the backing of their faith community.] It is also possible for a couple to be legally divorced but still spiritually married, if their religious organisation rejects divorce in their circumstances or in all circumstances- or legally married and spiritually never married e.g. if a couple are within the prohibited degrees of consangunity such as first cousins for faith purposes but not considered to be too closely related by the legal standards of their jurisdiction.
For the first few centuries, when Christians had no institutional power, most of their marriages were spiritual-only marriages, intended to place them in relation to one another, God and the church through sacramental means- not to gain a legal status predicated on being a married citizen of a specific country. The Roman Empire had plenty of secular ones. The distinction has tended to get lost since the age of Constantine because the Church hegemony led to either only religious marriages being available with no secular alternative in law or custom or, as currently in the UK CofE and other churches, the two being conflated in one event. Now that the legal definition is again set to differ from the one most religious people believe in, the parallel system could come back in.