The UK has many miles of coast. If you rock up on said coast it is pretty unlikely that you will be removed - amounting to an open border.
https://www.gov.uk/government/statistics/immigration-system-statistics-year-ending-june-2025/how-many-people-are-returned-from-the-uk
Quote from Government website above:
Data on returns relates to year ending June 2025 and all comparisons are with year ending June 2024 (unless indicated otherwise).
6.1 Small boat returns
Small boat returns have increased since 2018 but have fallen in the latest year.
In the year ending June 2025, there were 2,330 returns of people who had arrived by small boat, 7% fewer than the previous year.
Of the small boat returns in the latest year, 75% were of Albanian nationals (Figure 5). Turkish and Iraqi nationals are the two next most common nationalities, together accounting for 12% of small boat returns.
A total of 6,313 people who arrived on a small boat between 2018 and the year ending June 2025 have been returned from the UK, representing 4% of the total number of small boat arrivals over the same period.
https://www.gov.uk/government/statistics/immigration-system-statistics-year-ending-march-2026/how-many-people-are-returned-from-the-uk
This release goes up to the year ending (YE) March 2026. The “year ending” period includes the 12 months up to and including the YE month. For example, YE March 2026 includes the 12 months between 1 April 2025 and 31 March 2026.
In the YE March 2026, there were 2,750 returns of people who had arrived to the UK by small boat, 16% more than the previous year. This increase is partially due to the UK-France Agreement on the prevention of dangerous journeys which came into effect in August 2025 following the signing of a treaty by the UK and French Interior Ministers. This treaty enables the return of arrivals from the UK to France, with an equal number of individuals eligible to come to the UK through a new legal route.
A total of 8,427 people who arrived on a small boat between 2018 and the YE March 2026 have been returned from the UK.
END QUOTE
The report also states:
Afghan and Sudanese nationals were the second and third most common nationalities respectively arriving to the UK by small boat in the YE March 2026. However, returns to both countries are currently on hold. Iran (the fourth most common small boat arrival nationality over the same period) and Syria are currently not accepting enforced returns. See Home Office ‘Country returns guides’ for more information on the returns process for individual countries of origin. Despite this, 128 Afghan nationals and 93 Syrian nationals arriving by small boat with no right to be in the UK were returned in the YE March 2026.
.....So basically if you arrive on our coast from Afganistan or Sudan you are not going anywhere. An open border.