I acted as a party rep at the polling station a few years ago so hope I can fill in a bit of info.
It's quite normal for parties to ferry people (usually elderly party members or pledged supporters with mobility problems) to the polling station and while they can assist them into the actual building, as long as the voter is left alone in the voting area, that's fine.
Someone might ask who the particular party candidate is, and you can tell them, but you can't order them how to vote. You fully accept that while someone might have told you they support 'X' to obtain a lift, at the end of the day they may go into the booth and vote 'Y'.
I think the only problem that could arise would relate to a voter who lacked capacity, and was being pressured, but I've never come across that. I'm sure it happens, but hopefully not very frequently.
Parties do ask to check your voting number on the way out - they'll note it down and compare it against the canvassing returns. That way, if you've pledged to vote for them, they won't bother you later in the day when 'knocking up' their promised votes. If you've told a canvasser that you really really love that party and you are definitely voting for them in order to get rid of them, hand your number over or you risk having a desperate party activist ringing your doorbell at 7pm, to try and get you to vote!
Funnily enough despite often VERY bitter rivalry on the campaign trail, it's standard practice for parties to share those numbers with each other at the polling station. The campaign is over, everyone is tired, you all want the best data possible for the evening chase-ups. So if you give your number to the person with the Labour rosette, they'll happily share it with the Tory or Lib Dem person as soon as you're gone. Despite the fact they've spend the previous 6 weeks campaigning to crush them into the dirt.
Party reps can stand within the polling station building, but must be well away from the voting area. So in the entrance area basically, or just outside. They must be clearly identifiable as party members (usually they are wearing big rosettes). You do not have to give them your number, but they'll be very grateful if you do. If you feel they are getting a bit close to the voting area, or are a bit pushy you can alert the polling station staff who will have a word.
I did enjoy it and the local party I was electioneering for had a brilliant election weapon - a member with a Porsche 911 Carrera, who did driver duties every election day. We'd get calls from very elderly ladies weeks in advance checking that the dashing young Dave (he was in his fifties
) would be picking them up to vote. God forbid you offer them the alternative of the other party helper with the comfortable people carrier. They may have been 95, with terrible arthritis and a recent double hip replacement and have to be virtually crow-barred out of the car on arrival. They were going to vote in style! 