Don't use cold-callers. They'll rip you off. Find a reputable local installer.if considering solar who is good to talk to/buy from? Ive had a few cold calls on the landline that did not inspire much confidence and dont know anyone local who has taken the plunge, except for the one unfriendly neighbor.. (took a parcel misdeleverd to me round b4 christmas and got shouted at "Why u sending my wife pictures of your front door??" When it was obviously the delivery driver who mis-delivered!! ho humm tldr not asking him..)
When you specify your system, make sure you get an inverter of at least 5kw.
Get as many panels as will fit on your roof, not just a set amount. The more the better.
Ask about how you can get electricity during a power cut because the default situation is that you don't get it even though you're generating and/or have it in your batteries.
Get the biggest batteries you can afford.
Try to get an inverter/charger that can take an input from a generator to charge the batteries, or install a switch that can switch between mains and generator 230v charging.