Guidance for Candidates and Agents at UK Parliamentary by-elections in Great Britain

The earliest you can officially become a candidate is on the date that the vacancy occurs.

For example, where the vacancy is due to the resignation of an MP, the vacancy occurs when they take up a disqualifying office under the Crown. This is because an MP cannot resign, but their acceptance of the office of steward or bailiff of Her Majesty’s three Chiltern Hundreds or of the Manor of Northstead, which are offices under the Crown, would lead to a disqualification and, consequently, a vacancy.

Where an MP has died, the vacancy occurs on the date they died. 

You will officially become a candidate on the day the vacancy occurs if, on or before this date, you have already declared yourself a candidate at the election (or another person has declared that you are a candidate).

If, after this date, you or others declare that you will be a candidate at the election, you will become a candidate on the date such a declaration is made, or on the date that you submit your nomination papers, whichever is earlier.

Once you have officially become a candidate, you are entitled to a copy of the electoral register.

You are also entitled to a copy of the lists of absent voters for the constituency you are standing in. 
In addition, you can use publicly funded rooms and schools for public meetings.