This is a good point! But it's worth considering that there isn't a linear progression from hunting and gathering to agriculture that's bound to occur wherever it's environmentally possible - agriculture is just one cluster of ways of living, among many potential ways of living. It's possible (I'd say likely) that hunter-gatherers in Britain we're aware of continental agriculture for a long time without putting it into practice themselves. Both hunting/gathering and agriculture require a lot of specialised knowledge, and it wouldn't necessarily make sense for hunter-gatherers in Britain to try to emulate agriculture even if they were aware of it as a concept. In many ways, relying on agriculture is less secure than relying on hunting and gathering - it usually involves specialised knowledge of only a few staple species (of crop or animal), and if something in the environment causes the staple species to fail one year, you're in big trouble. Hunting and gathering involves knowledge of a wide range of environmental food sources, and if one or two of them fail, you can just increase your reliance on others for a little while to compensate for it.