I had looked there but they didnt list anything that size and i was a bit put off to see that 100mm wide 12mm thick flat at a length of 250mm was £43.35 with the vat and delivery
Try Orion Alloys Ltd , Harlow . they will sell small quantitys , but I would say it wont be cheap , they have to cut it and post and package , all adds up !
I needed a bit about 70mm x 70mm and got it from en ebay seller of stainless offcuts. They advertised they will cut whatever you need, I think it paid about £6 for a bit 6mm thick.
Can't recall the seller but have a search, for a one off it's probably not a bad bet you'll find a bit for a reasonable price.