According to the Forced Marriage Unit (FMU) a forced marriage can be defined as one where: 

one or both people do not (or in cases of people with learning disabilities, cannot) consent to the marriage and pressure or abuse is used. It is an appalling and indefensible practice and is recognised in the UK as a form of violence against women and men, domestic/child abuse and a serious abuse of human rights.” 

Forced marriage is sometimes confused with arranged marriage but the two are very different. A forced marriage is one where there is a lack of consent from one or sometimes both parties. There is usually an element of threat and coercion with forced marriage and this can come from one or both families involved. This is vastly different to an arranged marriage, where both participants give consent and enter the union willingly. 
In cases where the brides / grooms (or both) change their minds and experience threats, intimidation or any other form of force, what may have started as an arranged marriage, would then be considered a forced marriage. Also, cases where people are taken abroad without knowledge of a wedding and feel unable to refuse once out of the UK, would be considered a forced marriage under UK law. Not consenting to a marriage, under UK law is illegal and any such marriages would be considered forced. This would also be the case if someone consented to marriage because of fear or under duress. This is not true consent as the person consenting has not made that decision freely. 

Forced marriage sometimes involves people being taken out of the country to marry. This may be against their will or sometimes, under the assumption that they are visiting family, going on holiday or visiting a sick relative. In these cases, victims may willingly leave the country with family, only to find that a wedding has been arranged abroad, without their knowledge. This can add complications around access to support and finances, if for example, the victim cannot return home, is being held against their will or flees the marriage/situation whilst abroad.