I have a zone 1-3 travelcard loaded on my Oyster card, and want to travel
from Clapham Junction to Kew Gardens. This involves getting a train to
Richmond then changing for Kew Gardens.

However Richmond is in zone 4, which I have always assumed means having to
buy an extension ticket (for something over a pound). This seems like an
unreasonable expense & hassle, since I don't want to go to zone 4 or to
Richmond, it just happens to a necessary one-stop detour on the way to Kew

Does anyone know whether I can (legaly) travel through Richmond without
buying an extension, since my starting point & destination are bot covered
by my travelcard (and I don't pass any ticket gates, just walk from one
platform to another)?

Also, with Oyster if I loaded on some prepay as well, with the intention of
it covering the extension part, how would I go about doing that since I
don't touch in/out at Richmond?

Thanks
Tim