Congratulations 
You can get married anywhere in the country (or pretty much, the world).
You will need to give notice of your marriage to a registrar local to your home address. If you are having a civil ceremony, you will also need to tell the registrars in the area where you are getting married so that they can come out on the day and conduct your ceremony (and so you can sign the register and get a marriage certificate).
For the smallest and simplest of ceremonies, you can have a statutory ceremony. This will cost £50 for the ceremony and £70 for you both to give notice. The local authority where you want to get married must provide statutory ceremonies at a set price, but they might have some limitations on where and when they can happen.
Most local authorities will also be able to offer you registry office/community venue ceremonies. These still tend to be on the small side, but are a relatively cheap option. The ceremony will be slightly longer than the statutory one and might be held in a more attractive location. Once the ceremony is over, you can head off where you like for a meal or party or whatever you fancy.
Then you have all your approved venues, the hotels, stately homes etc. The sky is pretty much the limit in terms of guest numbers, food, partying etc. But the local registration office will still need to send out registrars for your ceremony and they will also license all the venues.
So...the best thing you can do is go on your local council website and look for Registration Services. They will either have info online, or you should be able to phone them and get them to talk through your options in your local area. Or, if you have chosen a venue outside your local area, get in touch with the Registration Services for that area (the venue should be able to put you in touch).
As I mentioned before, your notice of marriage is the big legal requirement for getting married. You have to attend an appointment in person with your local registrar with all your paperwork proving identity, nationality, address and that you are free to marry. This appointment must be at least a month before your ceremony date (assuming you are both UK or EU citizens) and can be up to a year before your ceremony. You have to be able to tell the registrar the name of the venue you are getting married in (even if the date is uncertain). The information is publicly available for the month after you give notice.
Sorry for the huge post. I hope it helps a bit.