HS, the difficulty we now face is exactly the same weakness that the Soviet Union faced 50 years ago and they reinforced their air policing with missiles. Remember Mathias Rust, now if anyone should have been intercepted it was he.
Now supposing we were warned of the departure of a light aircraft from Europe and were able to observe it approaching the UK at low level, then what? Would we scramble a QRA Interceptor?
Let us supposed we decided that option was too expensive and instead were content to track the aircraft overland. We see it alight at some farmer's strip or disused airfield - or even an active RAF airfield out of hours (it has been done) - now what?
We notify the civil police who send a car hot foot to the landing ground. Would they be in time? They can't stop hare coursers so what chance immigrants?