Honestly not much, but voter fraud is unheard of in the UK and the UK doesn't have a universal ID card. There is no legal requirement for a UK citizen to carry or have any form of official ID. Its just very impractical when trying to access some public and private services, but its not uncommon for poorer people to lack forms of ID. There isn't a mainstream racist/classist attempt to disenfranchise poor people in the UK by using imaginary voter fraud as justification (there are some accusations with regards to disenfranchising university students and young people that move around a lot, but that's an issue of ease of registration).
Also you need to consider:
1) Those kinds of people are probably not registered to vote. Voter registration is not automatic in the UK. It takes 5 minutes with an online form, but some people are just that disengaged.
2) The 'wards' in which you vote are often quite small and in close proximity, so it might be a little hard to figure out which polling place your friends are registered at if you haven't received their polling notification card. You can get this information, but its a small barrier.
3) The 'wards' have an average population of a few thousand, so there is a risk that an official might recognise you as trying to double vote, but I can't imagine how they might confront if they aren't 100% personally certain.
None of those are strong guarantees. If evidence of wide spread fraud were to come to light, party policy ID requirements might change, but to date no party is seriously considering introducing them. Past governments have repeatedly tried to introduce a universal ID that could be used in part for voting ID, but each attempt has ended in failure due to public backlash over civil liberties concerns.
https://en.wikipedia.org/wiki/Identity_Cards_Act_2006