If potentially earning a £26.5k salary 3 years post graduation is alarming to you, you best not pursue a career in engineering!
Like others have said there is large overlap between universities, within the quoted average starting salaries etc (remember many people are excluded from those statistics as well). It's not as simple as predicting your own/an individual outcome from them. Where you studied is only one of many hundreds of factors affecting employability, as well as factors from a hugely stochastic uk graduate engineering job market.
One such factor might be mechatronics being a more practical qualification whereas statistically more electrical/mechanical will suit higher paid management roles or be in the 60% that land grad scheme places. Many, many practical graduate roles in engineering start below £25k and are unlikely to progress much past £30k after several years. Even some grads from top 20 world institutions take years to find a graduate position. So I think you may be misinterpreting those results as salary at 3 years post graduation is not necessarily 3 years of experience.
In any case the statistics the universities show prospective/current students are always going to be tweaked to whatever looks best for the university. Contrary to what the uni's would have you believe, most employers aren't going to be familiar with differences between all the "electrical and mechanical", "aero-mech", mech-with... etc. It's all simply marketing to get you to choose their course. You are the product that the university is consuming here, not necessarily making you a product palatable to employers.