The Commons Library has produced a comprehensive briefing on Constituency casework: immigration, nationality and asylum.
You can find it here: https://researchbriefings.files.parliament.uk/documents/SN03186/SN03186.pdf
Here is the document’s summary:
Members of Parliament handle a high volume of immigration, nationality and asylum enquiries from constituents. Many of them are straightforward and can be answered using information that is readily available, for example on the internet, or by using the Home Office’s dedicated MPs’ correspondence channels. Others are more complex and require specialist advice from a solicitor or professional adviser.
This briefing provides a short overview of the UK’s immigration and asylum system and basic British nationality law. It also outlines the legal restrictions on giving immigration advice, gives some suggestions on handling constituents’ enquiries, refers to some sources of information that may be helpful for straightforward questions, and highlights training courses
available to Members’ staff.
It is important to remember that immigration advice is heavily regulated. There are legal restrictions on giving immigration advice and providing immigration services. Aside from this, immigration law is complex. If in doubt, you should recommend that constituents seek advice from a specialist solicitor or adviser. Encouraging constituents to access appropriate professional advice is more helpful than trying to be a substitute for it.
1 A brief tour of the UK’s immigration system
1.1 Relevant Government departments and agencies
1.2 An overview of immigration, asylum and nationality requirements
2 Dealing with constituency casework
2.1 Important warning about giving constituents advice
2.2 Useful initial information to get from the constituent
2.3 Contacting Home Office officials on behalf of constituents
2.4 Making representations to Government Ministers
2.5 Training courses for MPs’ staff
3 Useful sources of general information
3.2 Departmental pages on GOV.UK
3.3 House of Commons Library website
3.4 Books and journals about immigration law
4 Organisations providing free/not for profit practical assistance
5 Non-government sources covering policy issues, international comparisons, etc.