In a private watercraft I have undertaken multi-leg sea journeys to and from the UK via other countries (France, Ireland) without going through any kind of border control.
Are movements of this kind monitored?
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Through Her Majesty's Coastguard, the Maritime and Coastguard Agency operates some vessels that have radar. Its vessels range from rigid-hulled inflatable boats to five cutters it claims are operated on a 24 hour day, 365 days a year basis (although at times some of the cutters have not been available page 59). The MCA also operates some helicopters and fixed wing aircraft for surveillance as well as search and rescue.
Border Force has a total fleet of five cutters and six Coastal Patrol Vessels (CPVs) too - these have radar - and a number of smaller vessels. The Navy also patrols the coast but it's difficult to find information on schedules and vessels.
According to a Policy Exchange report published in July 2018, Dover is the only place that has coastal radar. However, port authorities have radar and there are smaller radar systems dotted around the coastline, which can detect targets as close as the adjacent beach or up to 15 nautical miles.
In September 2018 the Sunday Telegraph reported that the Home Office has invited tenders for development of a "super radar system" for the Kent coast. This must be "able to identify and track small boats, but mainly RHIBs."
There are various difficulties with using radar to spot a vessel, from its characteristics (including size and shape) to radar clutter, which can mask its presence. Despite radar systems, trained radar operators, radar reflectors, AIS, lookouts and innocent intentions, sometimes even container ships and fishing trawlers fail to see each other and collide. Or ferries and pleasure yachts (see also and also).
The Government recommends that "Yachtsmen permanently install, not just carry on-board, a radar reflector or RTE that offers the largest Radar Cross Section (RCS) practicable for their vessel". The Government also says it's "essential for skippers to be aware that, notwithstanding the type of radar reflector fitted, in certain circumstances their craft may still not be readily visible on ships' radars."
So you can see that even if you were trying to be obvious you may have been completely missed. Or you were spotted and deemed not to be of interest.
The Policy Exchange report says "In the absence of any data collection about small boats arriving in the UK, is there some consistent surveillance of the coastline itself? No, it is simply too big." But "crossing the channel in a small boat is a hazardous exercise that requires traversing one of the busiest shipping channels in the world."
There is radar for traffic control, especially in crowded areas like the English Channel. It is mostly concerned with big ships, so smaller ones may be able to evade it, especially in high sea states.