I found this quote on the following link:
http://www.weddingguide.co.uk/articles/legal/englandwales.asp"Marriages can only take place in authorised premises where the marriage can be legally solemnised. Venues include register offices, premises that have been approved by the local authority (for example, stately homes, castles and hotels), churches or chapels of the Church of England and Church in Wales, and other churches that have been registered by the registrar general for marriage including naval, air force and military chapels."
So, I would say yes and no. If the venue is a legally authorised premise, and it appears from above that some are outdoors, then you can marry outdoors. But, to just pick a spot, like someone's garden, I would think it would not be allowed. There must be someplace, like a registry office, that you can call and ask. Check the above site out. You might find more info there. Good luck!