MUFC - my point is that there is already a lot of capacity on the London-Barcelona route. Yes, FR does the pile 'em high, sell 'em cheap model well, but they are not immune to basic economics - there's only so many people who will want to fly this route on a given day.
An alternative strategy might be to pick routes where the existing competitors are weaker and have fewer resources (Vueling in this case, which has huge overlap with Ryanair's chosen Barcelona routes) and more likely to cede ground after a year or two of competing with Ryanair.
Setting up a base in BCN also signals to Easyjet that if fares on the UK-Barcelona route get too profitable, Ryanair will get involved (thus destroying some of the profits Easyjet makes at BCN), without it turning into a damaging full-scale fare war.