I really dont think there is an average cost. Dont be swayed by what magazines say is the average cost of weddings. You can have a beautiful wedding from £1000 + and I know of people that have done it for even less than that.
It all depends on what you want from your wedding. First place to start would be to fraw of a guest list of the people you really want there. If you want work colleagues, neaighbours and great uncle fred that you havent seen in 15 thats great but obviously costs will be higher.
Have in mind what you would like as a venue - obviously theres going to be a huge difference in price from a castle/stately home to your local village hall. Also keep in mind your guest numbers when deciding on a venue. 200+ guests wont fit into a small country inn, whereas 20 guests would look lost in a huge mansion.
Also decide what you want catering wise. Lots of ladies want just an evening buffet maybe with an arrival drink and then cash bar, helps cuts costs down dramatically from a full whistles and bells wedding breakfast, champagne drinks packages and evening buffet.
You dont have to the whistles and bells for it to be beautiful and perfect. Once you have in mind guest numbers, type of venue you want, and type of catering you want then I think you can start working out a realistic budget.
If your budget isnt huge but you still want all the little extras, there are lots of places that can help with making your own invites/favours etc... If you have a hairdresser/photographer/cake maker etc... in the family ask them to help you out.
Bear in mind like many of us your budget may change, due to circumstances or the need to have "extras" as long as its realistic for you to save whatever amount you decide upon and dont get yourself into unneccessary debt, you should be fine. xx