The best thing to do would be to check out a couple of magazines e.g. What Hi-Fi and the like. Some leads cost £50 and can be rubbish, but sometimes manufacturers sell one for £20 that is amazing value for money and makes a real difference. You don’t really know until you use it but the magazine reviews do at least give you a fighting chance.
Don’t assume more £ equals better, its not always true. I would always make a point of not going to places like Currys/Dixons/Comet etc as they tend to sell leads from big names that simply lood nice and shiny with weird colour leads but may not be better at all.
It SHOULD make a difference, but choose wisely!