How I get my logbook evidence (e.g aerotow and winch launch) certified when I have not been a member of a BGA club for over ten years.
You ask someone. There's nothing that says you have to be a member of the same club as the certifier but it will undoubtedly help if they know you. You are not asking that they certify your entire logbook, just that you have flown the minimum number of launches to qualify for a particular launch endorsement.
It may be possible to get a sailplane licence without any launch endorsements. On the power side, a pilot can still apply for an (A) licence even if he hasn't flown for years. His ratings will have lapsed so his new EASA licence won't have any ratings on it, he will have to do further training & testing to add the ratings but at least he will have the licence. It may be possible to do the same with the sailplane licence and launch endorsements.
The BGA conversion route is being negotiated with the CAA for the benefit of the members of BGA clubs. There's nothing to stop you applying directly to the CAA using SRG1104.