Deep States everywhere love to cheat on election day. It’s in their DNA to do so.
Beyond that, one problem is that the UK has a lot of political parties. And it only takes a plurality to win a district election. There have been cases where Labour wins a seat with only 1/3 of the vote.
The remaining votes are scattered among the many other parties.
They can pretty much get 1/3 of the vote with Marxists and Muslims living in the district.
Regards multiple candidates causing loss, It is what all parties do, with candidates and ideas/causes.
Always attempt to cause disarray by fomenting disagreement and injection of half truths while attempting to look like wind driven snow.
Having multiple weak candidates on the other side always helps, sometimes needing a shill to complete the steal.