I've just googld it and you can get married anywhere in Scotland but in England and Wsles it must be a registered venue:
"A marriage can take place in:-
- A Register Office
- A church of the Church of England, Church in Wales, Church of Ireland, Presbyterian or Roman Catholic Church in N. Ireland
- A synagogue or any other private place if both partners are Jewish
- A Meeting House if one or both partners are either members of the Society of Friends or are associated with the Society by attending meetings
- Any other religious building provided that the person marrying the couple is registered by the Registrar General - see under Religious marriage ceremonies (England and Wales only)
- Premises approved by the local authority (See below)
- A place where one partner is seriously ill and not expected to recover
- The home of one of the partners if the partner is housebound, for example, has serious disabilities or is agoraphobic
- A hospital, if one of the partners is unable to leave or is detained there as a psychiatric inpatient
- A prison, if one partner is a prisoner.
Local authority approved premises
Local authorities in England and Wales may approve premises other than Register Offices where civil marriages may take place. Applications for approval must be made by the owner or trustee of the building, not the couple.
The premises must be regularly open to members of the public, so private homes are unlikely to be approved, since they are not normally open to the public. Stately homes, hotels and civic buildings are likely to be thought suitable. Approval will not be given for open air venues, such as moonlit beaches or golf courses. Generally, the premises will need to be permanent built structures, although it may be possible for approval to be given to a permanently moored, publicly open boat. Hot air balloons or aeroplanes will not be approved."