It’s not rocket science and there’s no jerrymandering...
The British parliamentary system of government is rather simple. There is no vote for a party, you vote for a candidate that represents that party in the constituency where you reside. The candidate with the most votes within each constituency is declared the winner of that constituency seat..
The constituencies amount to the following:
England 543
Scotland 57
Wales 32
Northern Ireland 18
There are 650 constituencies in total and the party with 326 of more seats forms the government, and the leader of that party becomes the Prime Minister... If no party has 326 seats in parliament, you then have a minority government and coalitions between parties will be the usual result. The leader of the party with the most minority votes is typically chosen as Prime Minister.
Unlike the United States where 2 parties dominate your political elections, there are several parties in England and all of these parties suck up votes from the main contenders... The Conservatives and Labour.
At the end of the day, vote totals are of no consequence in the over all seat count... The seat count is based on individual constituencies. So while Reform may have received the 3rd highest vote total... That total is spread out over 650 constituencies throughout the UK.
Do you think that results with appropriate representation in Parliament?
In some ways it's like our Electoral College where the Founding Fathers wanted to ensure that the President had widespread appeal and that a super concentration of people in one area wouldn't necessarily win.