Where were you thinking you would go? I would imagine MK is relatively easy for people to get to by car or train etc. The architecture is supposed to be quite good (I've never been. I've driven through quite a few times however).
I've organised Oxford and London trips before. It's a case of suggesting a venue and a couple of dates and seeing who bites. Then pick the date most people can make.
Oh! And make sure there's a good pub for something to eat and a drink.