Although marriage was legal at 12 for girls you still needed parental consent under 21 years of age after the 1753 marriage act; No marriage of a person under the age of 21 was valid without the consent of parents or guardians. Clergymen who disobeyed the law were liable for 14 years transportation.
There seems to be some confusion about getting married without consent, possibly caused by the different rules between marriages after banns and marriages by licence.
First it is and always has been perfectly legal to marry after banns without parent's or guardian's consent “unless such Parson, Minister, Vicar or Curate shall have Notice of the Dissent of such Parents or Guardians; and in case such Parents or Guardians, or one of them, shall openly and publickly declare, or cause to be declared in the Church or Chapel where the Banns shall be so published, at the Time of such Publication, his, her or their Dissent to such Marriages such Publication of Banns shall be absolutely void.” (An Act for the better preventing of clandestine Marriages. [1753.] Section III).
Alternatively All Marriages by Licence require the consent of the Father or if he be dead the guardian or if there is no guardian the mother of the minor, unless the minor is a widow or widower. (Above Act section XI).
The above law requiring consent by licence lasted for about 70 years but I do not have the precise details to hand at present, but I think it was repealed in 1823.
Cheers
Guy