Hundreds of foreign murderers, rapists and child sexual abusers are being allowed into Britain from overseas, new police figures show. The violent offenders are exploiting “weaknesses” in Britain’s visa system which enable them to enter the country in the face of post-Brexit rules which allow the Government to ban any foreign criminals who have been jailed for more than a year. The criminal migrants are allowed to declare themselves whether they have committed serious crimes to secure a UK visa – and can only be exposed if they’re arrested in Britain. Only once arrested can UK police and law e…