The problem is say I decided that I would vote labour the Tory majority where I live is huge they poll about 52% labour gets 20 something. So for 48 % of the electorate in terms of influence they have nothing. In an extreme circumstance labour could get a huge majority of the overall votes but the Tories could get a landslide victory.
The system is hugely flawed and leads to apathy amongst voters. Also if you live in a poor part of a well off constituency you're doomed for life on local issues. And vice versa which goes without saying.