To convert you will need Bronze badge + cross country endorsement & certified logbook evidence of previous launches to gain equivalent launch rating (& a medical of course.) It is likely that the person certifying your logbook evidence & signing the conversion form will have to be the club CFI (ie head of training.)
There may or may not be a currency requirement. I'm betting not, & that you would be issued with an LAPL(S) or SPL but that you would have to fulfil EASA sailplane recency requirements before exercising the privileges.
If you don't manage to convert in time you can still fly EASA gliders as an unlicensed student at an ATO, effectively a BGA club. You would be under the supervision of an instructor, ie local soaring in the same way as a power pilot does solo circuit practice. Your power licence will give you about half off the 15 hours minimum training to get a sailplane licence. Some exams are common to all aircraft types, you would have to take the sailplane versions of the others.
Once you are licenced there would be no requirement to operate within a BGA club. You could self launch off a farm strip or aerotow behind a qualified tug pilot if you wanted to, but the rolling recency requirement, including two training flights with an instructor in the previous 24 months, would still apply.