As far as i am aware, most diy stores sell 18mm mdf but not 25mm. I think even still mdf is sold in 8foot by 4 foot boards, and not any smaller. I buy my mdf from james latham, but again in 8 by 4.
Having said that, there may be someone close to you who may have spare. I currently havd bits about the size you are talking about left over from jobs, but i live north of bath. If you ever pass by, i could let you have some.
Cheers
Nick