The biggest factor in the quality and life of the installation in the quality of the installer. Personal recommendation is a great help. Otherwise decide on a good-quality boiler (combis are more complicated than conventional boilers, so have more inside, so go wrong more often) with a long manufacturer's guarantee, then search their website for approved installers (this usually means they have been on the manufacturer's free training day) who are in your area. Phone a few, and say "do you repair XXXX boilers?" (because you don't want someone who will just bung it in and run off, but someone who provides a full service).
BG will always be the most expensive, but at least you know they won't go bust or run off with your money, and you have someone to complain to. If you have an old existing system then it may well have a certain amount of sediment and sludge in it. This must be flushed out with chemicals and a special machine, so you should expect this to be (1) included in the quote (2) visibly performed before the new boiler is fitted. It takes several hours to do properly, imcluding turning all the radiators off and on, and running hundreds of litres of dirty sludgy water down a hose into a drain or WC.
Usually a system filter is fitted nowadays to capture any future sediment before it can settle into a blockage. After fitting, the system should be flushed again, and an anti-corrosopn chemical added on final fill. This will be specified in the manufacturer's instructions. Manufacturer's guarantees don't cover problems caused by dirt, sediment or corrosion. They sold you a clean boiler, and they didn't cause the dirt.
If you don't already have them, it is very helpful to fit Thermostatic radiator valves at the same time, and to replace any old or worn working parts, and fit larger radiators in rooms that are not warm enough.
You can see here that there is a lot of extra work on top of just unhanging the old boiler and hanging up the new one. Anybody who does the job in a single day will not have time to do a good job, but can charge an attractively low price and make a substantial profit.