The problem is, even if you do intern all illegal arrivals there is always some do-gooding bastard making noises trying to get them out. The Detention Centres in Australia were deliberately sited in remote areas but the press and the rent-a-rabble set up a semi permanent presence outside the gates. This encouraged the inmates to stage protests and riots, which in turn led to the rabble saying that the centres were a dangerous environment and must be shut down.
Thankfully it seems the well advertised existence of these centres has had the desired effect as illegal arrivals are well down on previous years, also helped by changes to the immigration zone and increased sentences for the crews of the people smuggling vessels.
In the last few years the Australian Customs Service has started using large, armed patrol boats ( 7.62 mm GPMGs) to supplement the RAN patrol boats and Coastwatch aircraft that used to have the entire burden of patrolling the northern approaches. Perhaps the UK could do the same?