If you can build free delivery (and free returns) into your business plan do it.
But make sure you promote it so it’s visible on all pages of your website. This is because Free Delivery is one of those things that tips the balance towards a purchase with the undecided.
Free P&P is a good promotional mechanic. If you are selling large ticket items at high margins then you can probably afford to have it permanently. If not then most businesses have a threshold for free delivery and adjust that threshold to drive incremental business.
There is a difference between audiences that you need to take into account. Surveyed, women don’t mind reasonable shipping charges. Men on the other hand are more likely to purchase on free delivery. If you have a large number of bargain hunters on your website, free delivery will certainly increase your turnover. If you offer luxury goods with a high entry point (>£500) shipping charges are less of an issue for this audience. Free P&P is a mixed bag, I would use it as a promotional tool and only install it permanently after doing persona development and on-site surveys to see who your main audience is.