Yes and no. The real weakness is lack of recording when you leave the UK. The last time I went abroad there seemed to be no identity check: passports were only

Yes and no. The real weakness is lack of recording when you leave the UK. The last time I went abroad there seemed to be no identity check: passports were only checked on entry. On the other hand it is impossible to get a job or rent decent accommodation without the right visas. It does not matter to illegal immigrants who work illegally for below minimum wage and sleep several to a room rented from a slum landlord, but I cannot see the sort of people who can afford overseas fees doing that. There are also consequences. I know someone who overstayed by a fairly short time (more due to being disorganised than anything else) who ended up unable to get a visa again for years. I do know someone who wrote a book (The Professional, by Ashok Ferry) that I think is fairly realistic about the desperate measures people can take to stay in the country, but it was unusual back then for people from affluent backgrounds to do that, and would be even more so now.