The length of the coastline around Britain is estimated to be around 12,429 km or 7,723 miles, plus the fact that there is nothing to stop small aircraft from flying in and landing anywhere. It is impossible to control all but a few of the most obvious targets.
One method of assessing a person's right to be in the country would be to have identity cards, as are obligatory in most countries. When the government tried to introduce them a few years ago there was a public outcry, led largely by the Liberals. The idea should be re-introduced, pdq.