In scotland, which is part of the UK, a marriage can take place outdoors (assuming no other impediment to the location). An emergency marriage is the same. Certain religious bodies may have more specific rules of their own, but the requirements of the law itself are minimal.
The Scottish process, whether for a religious or civil marriage, always entails the issue of a Marriage Schedule by the district registrar, which gives permission for the ceremony to take place between the named people, at the given place and time. Normally, the proposed marriage must be publicly advertised for 28 days, to give time for any objections to be registered, but this can be shortened at the discretion of the Registrar General for Scotland - see the Marriage (Scotland) Act 1977, s.6(4)-(4A). So an emergency marriage is on the same footing as any other, rather than having a separate procedure. The only difference is convincing the registrars to expedite the request.
It is also possible for the registrar to alter the date and location on an existing Schedule, if there are new justifying circumstances, such as a change in someone's medical condition.
As far as location, there is a difference between civil and religious marriages.
- Civil marriages must take place at an "appropriate place", agreed by the parties to the marriage together with the local authority (the council), or else in the registrar's office. This can be indoors, outdoors, on a ship, etc. It can be your own home or garden, but it can't be a church or similar religious building. Each council will have its own policies on what they think is appropriate, which becomes relevant if the desired place isn't commonly used for weddings, but a hospice garden should present no problem in terms of dignity or safety.
- The government imposes no rules on religious marriages as far as location, but leaves it to the discretion of the celebrant. The idea is that the state has decided some particular religious body is generally trusted to perform marriages in an appropriate way, and so its ministers are able to get on with it. The religion in question might have its own rules which would affect whether a minister/priest/etc. will agree to perform the ceremony. For example, while the Church of Scotland permits its ministers to celebrate marriages anywhere, at their discretion, the Catholic Church does not (but it does have well-established procedures for marriage when one of the couple is terminally ill, including dispensation from normal rules about the ceremony). Jewish ceremonies can happen outdoors - again, if the rabbi agrees - but likely under a chuppah; Sikh wedding logistics regarding the Guru Granth Sahib can be complicated even if the outdoors is possible in principle.
Further, the location on the Marriage Schedule is in the nature of a street address rather than anything more specific. A planned outdoor wedding can be moved indoors because of the weather without causing any problem with the paperwork. Similarly, as long as the registrar or celebrant is happy to do so, the marriage ceremony can take place anywhere at the agreed location, including outside. And if the location of the wedding is other than what was specified, that does not affect the validity of the marriage (per a reform in 1980, extending an original exemption so that all merely procedural defects are now covered). The celebrant may get in trouble for breaking rules - either with the state or with their religious authorities - but in terms of civil law, the couple are in the clear.