I think it's a crazy idea and you would regret it.
Why jeans and a t-shirt anyway? Why not a nice dress (or smart jacket and trousers for the men.) You can easily get a nice wedding outfit for less than £100. Less than £50 in some places.
Lots of couples I know - upper middle class, professional, quite wealthy/very solvent, are all going with the least expense they can possibly come out with for their wedding.
One young woman I know got her sister to grow sunflowers, her friend to take the photos, her dress from a quirky online company for £100, and had a carvery at the pub for a fiver each for their 30 guests.
Many of these couples now do the lot (excluding the honeymoon and rings,) for less than £700.
This isn't a 'race for bottom' type scenario; it's just that you don't NEED to spend £20-30K anymore, and quite frankly, unless you are a celebrity, spending that much smacks of desperation to impress, low self esteem, and insecurity. What are you trying to prove? 