I am a householder
try to get a personal recommendation for an installer, from a friend or neighbour and have a look at his system, but remember most householders won't be able to tell a good job from a not so good (until it goes wrong).
get a couple of comparative quotes, for the same thing. Write down you requirements in advance so they are quoting for the same thing. If you are re-using old pipes and rads it must include a powerflush (which is the best part of a day's work if done properly) and if the old system is dirty, a Magnaclean, Sentinel Filter of Spirovent or similar to catch existing dirt. This will add about £100 to the cost but IMO is worth it.
Your gas and electric suppliers will probably be most expensive but at least you have someone to complain to if not satisfactory and they may include a year or two's service contract in the price.
I chose a manufacturer's approved installer, local firm, corgi registered, premises less than a mile from, my house, well-established company, and it was still not very good.
Maybe I would have done better with a one-man band who did not employ less skilled workmen
As a householder, I selected a boiler with stainless steel heat exchanger, not a combi, simple design with very few internal parts on the basis that it had less to go wrong needing a specialist repairer, and with a long guarantee, and I still think that was a good plan.