corasri,
Not sure what trim comes in the UK versions, but I would hope it wasn't much different from the US - the cost to produce that sort of thing differently by country would hardly fit into the cost-saving business model.
I installed rear speakers recently (see my gallery for pics), using 1" thick MDF rings to mount the speakers. The rings mount to the plastic trim, with a hole cut through the trim to accommodate the speaker and wires. The ring is held on by 8 screws through the plastic into the rings, plus the four mounting screws for the speakers themselves. To get the trim off requires a bit of work - you need to remove or loosen all panels that border it. Other than that, and running the speaker wire under the carpet along the center hump to the back, then up to the speaker, it is fairly simple. Making the rings took more time than installing everything.