I guess the OP is takling about exit polling, or general local party election polling people.
These people cannot be inside the polling station, but to know where that starts is up to the presiding officer and might be a line within a room not the whole building.
They are allowed to ask you your polling card number and your choice of party, but it is up to your if you give them the info.
Inside the polling station only the registrar can ask for your polling number and they will give you the polling candidate sheet.
Candidates, with rosette can attend polling stations.
People working on polling stations can also talk to voters, but generally not until after they have voted.
THere are plenty of little laws around all this, if you are concerned go and ask for the registrar or presiding officer and ask them.