I'm in the UK too. Did you manage to follow a path to real kefir grains? If not you'll find some here. The best way I've found to culture veggies - apart from the rejuvelac I posted before - is simply this:
Chop up a red and white cabbage(saving the outer leaves for later), or any other combination, as long they're organic. I've used savoy, pointed, etc. Basically anything I can get.
Wash them when chopped and put in a big bowl and bash merry hell out of them with a rolling pin(fun!). At this stage I sprinkle with celtic salt.
When you've done this plop it into the jar pressing down with the rolling pin to ensure a tight pack.
In layers add caraway seeds, cumin, fennel or anything you fancy, but caraway and fennel are my favs.
You can also add layers of carrot, broccoli, cheese(not really, was just seeing if you were paying attention;-)) etc.
when it's about three inches from the top take the outer leaves, washed and with the hardest part of the stalk cut out, and pack down on top forcing the leaves down the side to pack the cabbage in. The point of this is to prevent bits of veg floating up to the surface which can compromise the quality. I usually use two leaves then pierce them with a sharp knife so gas bubbles can escape during fermentation.
Now you'll need a large stone - the kind you get from aquatic or garden centres is best. I scrub mine them soak in Colloidal Silver and then boiling water to ensure they're microbe free.
Plonk this fella on the veggies(it's important to have some room above the stone for expansion of the veggies. That's why it's worth getting the largest jars you can) as a weight. This is so important as the gases released in the fermentation process will cause the veg to rise up above the water line and mold.
Now drizzle pure water over the top of the mixture until it's an inch or so above the veg. you might have to go back later and top it up.
Seal the top and watch it for 3-4 days. 3 days gives a real crunchy texture, 5 will still retain some crunch but gives a fruitier, sharper tang(my fav).
If you see a bit of scum forming or it looks like the water level is rising too high feel free to open it up and scoop some out. As long as everything's clean you should be just fine.
One last thing. Do it with love and blessings and the difference will be noticeable.