you can usually rely on BG to be the most expensive.
I'd suggest you choose a good manufacturer, such as Vaillant or Worcester Bosch, and go to their website to see if they have a register of approved local installers.
When replacing the boiler in an old system, it is especially important that the system is thoroughly cleaned, usually by Powerflushing, which is half a day's work if done properly, and a system filter such as the Fernox, Sentinel or Spirotech fitted. The Magnaclean has a poor reputation for build quality and leaks.
Damage or blockages caused by sediment are not covered by boiler guarantees, since it was clean when they sold it and their installation instructions say the system must be cleaned.
Cloose an installer who does servicing and repairs as well as installations. Your friends and neighbours may have opinions on local installers.
Have your new boiler fitted where the condensate pipe can go direct into an internal drain or waste pipe. External pipes are prone to freezing in severe weather, which is exactly when you don't want your boiler to stop working. An installer who says it will be easier to run the pipe outside is an installer you don't want.