The English channel is one of the most monitored waterways in the world, last time I checked the amount of ships going through is 40 an hour every hour. With that amount of traffic obsessive monitoring is needed for safety.
How do you know "every little boat" gets stopped? Maybe we just don't hear about the ones that get through.
I could show you barren coastline of the UK and go look proof of piracy if I was using your naïve logic or I could use common sense and realise that a countries coastline is monitored.
Australia has a huge coastline but for some strange reason every little boat with illegals somehow gets stopped hundreds of miles before they get to the coastline.
It isn't magic its technology.
If you could sneak through on a boat to the UK then you wouldn't be bothering trying to sneak on to ferries and/or lorries.
There is a reason why illegals and smugglers try to move with and pretend to be travellers, as its easier than trying to fool the Navy and Coastguard.
Some people just try to sneak in through every way they can. And there's more than one way in to the UK, the less official, the better.
I guess the IRA wasn't a problem, huh...