Congratulations on setting your date!
So, you need to give notice between 12 months and 28 days before your actual wedding. This has to take place in your local registry office and is just a chat with the registry office people to make sure you aren't already married, related, marrying for a visa, that sort of thing.
Wherever you choose to get married, you will use the registrar from the local area. So get in touch with the local authority for the venue in Cornwall now to provisionally book your registrar on the day and time that you want. Is it a 2+2 legal ceremony you want? Bear in mind these aren't always available everyday - in my area they generally offer them on a weekday, and sometimes not a Friday. So bear this in mind, as if it isn't available on your date, you could either book the legal part in your local RO a day or so before you travel to Cornwall.
The other option if you want it all done on the same day (this is more expensive, but also what we are doing) - book the registrar to come to your venue to do the legal part basics (i.e. no vows, ring exchange etc). They then leave, and you can do your vows with your friend presiding. We have a celebrant to do a handfasting and ring blessing for ours. Just note that if there are any religious or spiritual connotations to your blessing with your friend, the registrar has to leave the premises first.
I hope this helps! X