Sounds like you are needing some guidance on this project. Forgive me if I am being overly didactic. I've done a number of root over rock bonsai, and it is not an overnight process and much more involved that just plunking a tree down on a rock. If you do that, the tree will most assuredly die. If you live in London, i'm guessing space is at a premium. I lived in a San Francisco for many years and didn't have a good place to ground grow trees. This is how I did mine with moderate success.
First and most importantly, pick a good rock. You can make changes in the tree, but the rock is always going to be the rock. Something interesting with crevasses is good. Second the rock should be fairly hard, skip sandstone or anything too crumbly. The tree should be fairly young. A 2-3 year old trident sapling would work. try to find one with a little movement in the trunk. When you choose your day to do the initial root work, give yourself plenty of time and have things ready. Roots can dry out, so be ready with a pot, soil, a good sized piece of screen, landscapers tape, mister, tools, wire etc. I had the best luck with using a cylinder of plastic screen that was large enough to encircle the entire rock. (Roughly the size of the pot and as tall as the rock.) You can use the type of screen that you'd use for covering drain holes. Don't plant it in a bonsai pot. Use a colander, a pond basket, or large terra-cotta pot. The good news is that you got a trident, which are great for root over rock because they havevigorous roots. Start by bare rooting the tree. Keep the roots long and choose where you are planning in placing the tree on the rock. Drape the roots in a natural, and interesting flow around the rock surface and hold them in place by using landscaping tie down tape. You know, the ugly green type. Make sure that the tips of the roots don't get covered with tape, you'll want those to grow down into the pot. When the tree is tied on to the rock to your satisfaction, plant it into the pot a little deeper than you want the final composition. Put your screen around the rock and tree, with plenty of room to spare on all sides. I usually did my cylinder of screen about the same as the diameter of the pot and tall enough to go and few cm below the soil surface all the way up base of the trunk. Covering the rock, roots and all. Use a loose, fast draining medium. Treat it as a new transplant and let is grow without any trimming for a full season. It would be best to just let is grow for a year, but if you are in a gut-busting rush, and if the growth is vigorous after a few months, vigorous, not just okay, you might start removing a little of the soil from the screened part, say, no more than 20% of the height of the entire screen. If the roots look strong and have thickened, leave it off. Repeat the process, a 6-8 weeks later, but keep an eye on the roots you expose. Easy does it. Slow and steady progress. If all goes well, you may have the roots uncovered halfway by the end of the season, maybe more. Trident maple roots grow fast, but they are kind of succulent when they are new. It takes a while for them to get sufficient bark to grow out in the open air. It is better to let the roots stay covered if you are not sure. If you leave them covered, they will grow faster.
The trees I grew like this are nice enough root over rock, they would be much better if I had ground grown them, but this way you can do it in pots. I eventually sold them all off except one.
I always made my root over rock trees with smaller air layers that had been off the parent tree for a year or more. If I was planning to use a layer for this kind of design, I would grow it is a taller pot so the roots got long and grew downward.
There are several books that demonstrate this way of doing the style, so look around to see what you can find before you start. There was a great article in Bonsai Today several years ago. You might be able to find that one and get a good tutorial. Good luck.