However, under the EU Dublin Regulation, we could send them back to another EU Country under certain circumstances, we are still bound by that until the end of the transition period. After Full Brexit in January we loose that lever, being no longer part of Schengen or Dublin III, nor have we yet established agreements or treaties with the EU/Individual member States ((other than the limited Le Touquet Treaty with France) and there may be some considerable time until we can. A No Deal Brexit guarantees that we we lose any mechanisms to return asylum, seekers or immigrants to the Continent.
It's not an easy one to solve.
No it's not. A fleet of floating fortresses armed with miniguns parked at the midpoint of the channel would be a good lever. And the ones that do get through can suddenly find themselves on a G4S RIB back to France. If they can slip through the French border inbound, we can do the same outbound.
You could have replaced the rest of that para with the words 'Black market economy in London'.So why do they come, well,
As for our return jihadi bride in the OP, she was born in the UK, and has British Citizenship. However the government has claimed she has Bangladeshi Citizenship, or at least the automatic right to which would make it easier for us to strip her of her Citizenship.
Bangladesh however are saying " Get bent, no she hasn't. Not our problem matey."
And as we can't legally rendered anybody Stateless we find ourselves in this long running legal saga of Yes/No/ Appeal.
We don't have Jus sanguinis (citizenship by blood) here,. We have Jus soli (citizenship by territory) but with massive restrictions, in this case "behave or you won't be allowed back in if you join a foreign terrorist group." Since we withdrew her passport, as we had every right to do, she now has a claim to Bangladesh via the father, so she isn't stateless.
We got our action in before Bangladesh did, so they are left holding the baby (literally) when the music stopped!