I don't think that's possible unless the venue has been given a special licence to hold weddings. Have a look on this website which lists the register offices in Cornwall and contact the nearest one to your future in-laws to discuss this with them.http://www.cornwallweddings.co.uk/registryoffi ces.shtm
Regretfully, no.
Firstly you can not get legally married in a garden. It has to be in a building suitable for the purpose.
Secondly, it has to be registered for that purpose and the general public also have to be allowed to get married in it if they wish to do so. Remember the problems Charles and Camilla had about Windsor Castle?
Sorry to dampen your idea. Your local register office will be able to explain better or try the GRO (General Register Office) website.
I got married in a different area, but first I had to register an address there, had to vote there also. I used my brothers address, so maybe you can do the same with your future mothere-in-law.