For a while, I've been looking for a combi boiler that's compatible with solar water heaters. I'd heard that most combi boilers weren't suitable, but nothing more specific. Well, here's one: the Ferroli HE S boiler range seems to be compatible with their [pdf] solar hot water system, and their underfloor heating system. They use a solar panel to heat a small tank of water, and use that tank to preheat water on its way in to the boiler.
They seem to have some other nice products, like their "'Plug and Play' AquaSolXS", which combines various parts of a solar water heating system with a hot water cylinder, to make an easy to install system. That should keep installation costs down, perhaps even component costs, too.