YT, yes that is a risk. And you are on your own in such a case. But, depending on the fare conditions of the long haul fare you may be allowed to easily rebook flights. Less flexible fares may allow a change for a fee. The more flexible, the more expensive though. If something does go pear-shaped make sure you contact the longhaul airline as soon as you can to advise them and sort out options. Try and not no-show, it gets more difficult to fix afterwards.
Check the fare conditions so you know what would happen if your outbound transport over the channel is delayed or cancelled.
Travelling the day before is wise, but it does add cost - unless you plan on sleeping in the airport! You will need a longer connection time - a minimum of three hours to be on the safe side.