@George , the days of pv being a good financial investment went out with the FIT payment. I was an early adopter so get the highest FIT rate, and mine took 8 years to payback.
I think your generation numbers look reasonable. I'm not very far from you, and my 4kw system averages about 3,800 kwh per year. So, the projections for a 5.4 kw setup seem OK. My shading, roof angle, and orientation aren't that different to your numbers.
As @Colonel Panic points out, the real sensitivity here is the usage. Without a battery system and the solar switch set up that CP suggests which add costs, you will be unlikely to get anywhere near those numbers. 50% would be high. Further, you need to factor in the inverter costs. Mine is still going strong, but average life is supposed to be about 10 years, and a good one isn't cheap.
Using 50%, a saving of just under £500 pa means 12 year payback not 10 - have they ignored the vat? However, see my points above on additional costs. And that doesn't include any cost of money - the return you might get investing the money elsewhere.
This doesn't really stack up as a financial investment, but if it is any consolation, your total installation cost is well under half what I paid in 2011. I personally like the green aspects, and being a nerdy numbers guy love keeping records which gives me hours and hours of spreadsheet fun. My illness extends that far that I could tell you my generation on any day since
I had my system commissioned.
If you are planning to stay in your home for a good while and can spare the cash, then I'd say go for it. But just don't make that decision based on financial returns.