The offence is defined as "custody or control of a false instrument", contrary to Section 5(2) of the Forgery and Counterfeiting Act 1981.
The actual circumstances relating to the possession of the false passport will have a considerable effect on the sentencing. The offence is 'either way', so it can be dealt with solely in a magistrates court (where the maximum penalty is 6 months imprisonment) or referred up to the Crown Court (where the maximum penalty is 2 years imprisonment).
A relevant case is that of R v Fabio de Oliveira, in 2005, where the defendant pleaded guilty to simple possession of a false passport. The sentence was 8 months imprisonment.
Chris