uncletone has decided it's time to step aside from being an Admin of the forum. Thank you uncletone for all your help and support over the years. Wishing you a long and happy retirement and hoping you don't feel the need to come back and use the slipper too much...
The most important factor is what will you house look like with solar panels fitted. If your fitting them to the front of the house and shuffling them around chimneys and velux windows it will probably reduce the value of your house. There's a house near me that looks shocking.
The panels with black edges are a lot less unsightly than the silver edged ones.
This site will give you an estimate of how much you will generate for your location and conditions (roof alignment etc).
Definitely see previous installations of the installer if possible. The one I went with though he was a smaller outfit than some of the others really understood the fine details from experience.
The unit that diverts spare power to the immersion heater is almost definitely worth it. The eco-system I bought into (Energeno / Optimmersion) also has plugs that will divert spare power to appliances (e.g. heaters), but it looks like they went bust recently. I guess other suppliers do similar but I haven't investigated.