Glowworm & Vailiant are essentaiily the same, different brands I believe. Both good reliable combi’s iin my experience, particularly the Vailant. We have had both a combi & system which were really reliable.
I take ir your on natural gas ?
Worcester & keston both really efficient but from what I gather a tad delicate.
Just make sure that you get a good ZEDBUK rated boiler with a boiler interlock & load compensate.