A wealth of statistics are produced by government departments, particularly the Home Office, on migration to and from the UK. If you're studying Refugee Law then there is undoubtedly some food for thought in these. There are also some useful facts and figures for those interested in related fields such as the use of migrant labour, the free movement of workers etc.
Here are a few sites to have a look at - this is by no means an exhaustive list.
Research Development Statistics on Immigration : These cover areas such as asylum applications, decisions and appeals, people refused entry to the UK and applications for an extension of stay.
Office for National Statistics Long-term International Migration series : Provides information on the flow of long-term migrants to and from the UK.
2006 Home Office report on Employers' use of migrant labour
In addition, it is possible for Birkbeck staff and students to access the OECD Migration Statistics, part of the Economic and Social Data Service (ESDS). These data include information on immigrants by age, gender and occupation as well as emigration rates by country of birth. To use these sets users must first register with ESDS - details available via the link above.