Yea I noticed this a few days ago :)
If I wanted a 10mm sheet of 6082 to fit my router bed that'd be £274+VAT ... ouch.
Still it's clearly good if you just want a small area.

It looks like their price for 6082 plate is about 23% more than the same grade flat bar. Still I think sometimes it will be worth paying that bit extra to get the size you need instead of buying 2m lengths.