In my humble experience, most light GA owners in the UK hand their aircraft to the maintenance company on the airfield where they are based for both maintenance and CAMO services.
I believe EASA did not think it would be this way but envisaged the CAMO system would work in the way it does for light jets for all aircraft. I know people who looked at offering stand-alone CAMO services to to light GA maintenance companies and owners but, in the end, every man and his dog applied for Part M Subpart G to get the "Subpart I" privilege to issue ARCs.
What UK is left with is a system almost as it used to be. Before Part M light aircraft engineers were approved to make a recommendation to the CAA to renew a C of A for another 3 years. All the CAA have done is give those same people the privilege to issue ARC's.
As in the old days, when an owner takes his aircraft somewhere else for maintenance, the log books follow to go into the office of the maintenance company. I know very few owners who, like me, keep their log books themselves (which is why so many aircraft have incomplete history because log books fail to be transferred, 8130s and Form 1s are lost.....) and even fewer who are competent enough to understand what is required.
The answer is, in UK anyway, to take your aircraft to the maintenance company and they will either be the CAMO or the ARC will be issued by them.