Sounds from your post about a 'local airfield' that you are talking about light aircraft work. In qualification terms there is a difference.
'Small' Aircraft radio work.
You need three things to repair light aircraft radio equipment on your own.
1. Some ability.
It may seem strange, but fixing light aircraft radio systems can be a challenge. There are several different types of radio equipment from several different manufacturers. If you have never had any guidance from somebody with experience in the job, starting from scratch would be an uphill struggle.
2. Test equipment and books
This is not cheap. Major pieces are (but not limited to) NAV401/2L and ATC600A minimum. Even some of the very lightest aircraft are having mode S transponders fitted which means your ATC600A will not be enough. Also you need all the books from the manufacturers for repair work and installations, with the update service of course.
3. A licence
'Light aircraft' work can still be certified using a CAA issued BCAR licence. Refer to the CAA website and look for BCAR section L. It's the 'R' licence you interested in. CAA Airworthiness Notice 3 also has relevant information. You will note an experience requirement, which may cause you some trouble. You will also need a type rating to the basic licence, paragraph 12.2 and 12.3 of CAA Airworthiness Notice 10 if you intend to work alone.
With the above three things in place you can be the 'specialised' subcontractor you referred to in your original post.
Large aircraft radio work.
Different game altogether this. You need to be working for an approved company to certify work on large (>5700kg) commercial aircraft. There is some complexity I won’t go into as it does not appear relevant in this case (existing CAA licence holders), but for this type of work you need a JAA (now EASA) JAR66 B2 licence. This is a big task. If you want details of how to achieve this please send me a private message and I will talk you though it on the phone.
Hope the above helps
Regards
The Doc