Aircraft avionic systems are maintained and certified fit for use by licenced aircraft maintenance engineers, who hold a B2 (avionics) licence issued by the CAA to the Joint Aviation Authorities i.e. European JAR66 standard.
This is a vocational qualification. A university or degree is not required, but the intellectual input required is still reasonably high; I would think you would cope with that. Much practical experience is also required. I expect you have looked at the CAA website?
Perhaps the usual route is to work on aircraft in an unlicenced role whilst studying and taking occasional short courses at one of the training schools.
However I myself spent two years at college full time ten years ago at what was then called Brunel College in Bristol. Now called Bristol College of Arts and Technology or something. I think they do short courses. Also Engineering Training College Oxford, very good; 01865 844253.
And I think there is still a college at Scone, Perth. I am vague about this.
Distance learning? I think they might do this.