Complementing koiyu's answer
Snow Leopard license/installation
Lion on a USB flash drive from Apple
I assume that amongst other things, this product will require neither pre-installation of — nor a license for — Snow Leopard.
Costs
If that assumption is true then, based on highlights from the press release, the USB flash drive is more economical than an upgrade:
£21.67 … Mac OS X 10.6 Snow Leopard
£ 4.50 … delivery
£ 5.23 … VAT
£20.99 … upgrade to Mac OS X 10.7 Lion
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£52.39
or
£43.99 … Mac OS X 10.7 Lion on USB thumb drive
License for Lion in multilingual Portable Document Format
After Lion is in installed, the license (EA0730 revised 2011-06-01) is amongst the documentation:
/Library/Documentation/License.lpdf