PV panel prices have fallen dramatically - to perhaps 1/10 or less the price per watt that they were just 6-7 years ago. They used to be the biggest expense on a system BOM. Not any more.
This has been due to a huge ramp up in Chinese production of panels. Almost all panels are produced in China now and they are almost without exception good quality with 25 year power production warranties (10-15 on workmanship). In short, PV panels have become a commodity item but installers have not adjusted their prices accordingly
Not sure about the situation in the UK, but in the US solar install companies generally supply the panels. Their system install prices have not dropped accordingly - IOW they are gouging consumers on their quoted PV panel costs.
If you want to save money, get a quote from installers with a breakdown of the BOM costs. Then offer to provide the panels yourself - if they refuse, see if you can negotiate on their quoted price.
To get an idea of the cost of solar PV panels bought in bulk - have a look at the
Sun Electronics website. They sell internationally from individual panels to bulk (pallets and shipping containers) with expected discounts for higher quantities.
To give you a concrete example, I just recently bought a pallet (27 panels) of Grade A ETL/UL certified 285 watt panels for $0.24 per watt (+$750 shipping) = $0.34 per watt delivered. Just 8 years ago I paid $4 per watt just for my panels!
Depending on the location - installation of the system + grid tie inverter + combiner, wires, breakers, etc should not cost more than about $2 per watt max. So adding in panel costs you should not be paying more than about $3 per watt total IMO (though I expect you'll be quoted anywhere between $4-6 per watt!)
Unless your install locations solar insolation is very poor - complete payback should be less than 10-15 years even without any feed in tariff.