Installed three weeks ago.
The UK has referral websites that promise three quotes. The two I tried were both poor - one got me a single reply with a sales visit but no quote, the second also a single reply with an on the spot quote from the salesman. Neither referral site contacted me again. Based on that experience, you're on your own.
I went for my only quote which matched what I was expecting. Installation took half a day with the only problem being that the original plan for 15 panels had to be amended to 13 as the roof was inches too short. That gave me a capacity of 3.4 Kw.
The panels are connected to an Inverter in the loft that converts from DC to AC and that joins the mains supply next to the existing meter under the stairs. A new meter records the panel output.
Right now (computer on) I'm using 0.6Kw. The panels generate 0.2Kw through thick cloud, 1Kw with thin and a maximum of 2.6Kw with a blue sky.
[added: maximum midwinter is 1.9Kw]
So where are the savings? The Government pays a subsidy for 20 years for every KWh produced, plus a lower rate for each KWh exported to the Grid. And, of course, the electric bill goes down. Estimates suggest a 12% return, ie the system will have paid for itself in around eight years.
One thing I looked into was batteries, but they are not cost effective yet. Like panels the technology is improving, and in a few years it might be worth adding them.