There is a pot of liquid plastic-type stuff in red color, so if you have an old SMA connector that you block off then you could dip it in.
The pot is intended for creating handles on household tools, etc. See
here.
There is a**broken link removed**.
I've never tried this stuff though, so I couldn't say how effective/ineffective it would be.
However, when I needed to protect an SMA, I used a normal SMA plug and removed the pin, so it was an empty shell.
However, it does need to be screwed, so it's not great.
Alternatively, on another (unrelated) thread, someone once mentioned
this site to me. They may
have something suitable. I've never used it though, but I guess they would ship to the UK.
EDIT: just saw your post. That looks a great site, very useful.