The total cost of the 6.25kWp system, in-roof, was around £8k fitted. The cost of covering that area with slates would have been around £2k (inc labour). The system is too big for domestic FIT, so is rated as a commercial installation, so a much lower FIT rate and we needed approval from the DNO to export more than 16A into a single phase (we will be running under a G59 approval, rather than the G83/1 small scale exemption that most domestic PV installations run under).
FIT isn't really a consideration for us, as the income from it is small. The main reason for fitting a big array was that it was the cheapest way to both heat our hot water and provide more energy (over the course of a year) than the house ever consumes. In effect we will be "borrowing" power from the grid in winter (and paying for it) but exporting power during the summer (and being paid for it). We will have an export meter as part of this install, as it's a commercially rated system (even though it's on a small 2 bedroom house).
There is no "masssive" FIT rate for us, as it's a commercially rated system and exported power is metered, unlike a domestic small scale system running under the G83/1 exemption.
The solar panels have a 25 year life and there is a lot of evidence to show that output degradation over that lifespan is modest (of the order of 10 to 20% or so, maybe 30% in extreme locations). The panels themselves are cheap, so replacing them wouldn't cost anywhere near the cost of installing them.
You can indeed use staged heat pumps to increase the sensible heat output from a solar thermal system. AFAIK there aren't any readily available systems like this, but it would be relatively easy to use a small water source heat pump and feed it from a buffer tank fed by a solar thermal system. This would then boost the sensible heat output temperature up to thermal store temperatures and allow sensible heat to be extracted even when the temperature of the solar thermal system output was well below that of the store.