I agree the posted article isn't particularly clear - found this one [1] on the Guardian that's better.

The understanding I get from it is that MasterCard could be liable under UK law now that the EU court has decisively shown the fees charged to have been anti-competitive. What's required for the UK claimants is to show that harm was caused to them by these anti-competitive practices and that redress is due.

The Reuters article makes it seem like "the fees were just too high" is the reason for this suit. A court-of-law decision that anti-competitive practices took place makes a lot more sense as a pretext for a claim like this.

[1] https://www.theguardian.com/money/2016/sep/08/mastercard-sue...