Local plumber's merchant, Vailant (not Valiant!) are great it's what I installed when ours packed up.
Regular or combination? You'll need to know the size which is determined by how much heat you want it to kick out which is determined by the total heat losses of your residence.
Modern units are, almost without exception, 'condensing' boilers and are more complex to install due to the extra waste pipe which removes the condensate.
The last time I studied the regulations it was perfectly legal to install your own boiler, as long as you do it in accordance with the manufacturers instructions, building regs etc. You are merely required to be a 'competent person'.