Many thanks for your kind replies, and although I confess I don't grasp some of the more detailed tech lingo, it sure gives me confidence that people here know their stuff!
I'm going for a Vokera Vision Plus 30C (combi-boiler), CH 25kW, DHW 31.6kw, Flow Rate 12.9L/min. My flat is 2 very large bedrooms, 1 very large living room, and 1 shower room. The estimated heating requirement at delta 50 is approximately 16kW (3.4m high ceilings!). So I easily could go for the 25C, which is CH 20kW, DHW 25kW, Flow Rate 10.2L/min. But I thought I would go for the better flow rate; my water pressure is approximately 20L/min.
According to the tech spec of the Vision Plus:
Vision Plus C Data Sheet
This does make sense to me. If there was no modulation, then I could see why you would want to limit the max heat output of the boiler. But with modulation, I can't see why you would do this. Indeed, in some circumstances you would want to boiler to go to max.
eg: You come back from holiday in mid-Jan, and it's minus 5C outside. The heating has been off for 3 weeks, and everything is stone cold. So you want to heat the CH water as quickly as possible, and the radiators are shedding large amounts of heat, as the rooms are so cold. The boiler can work at it's max, until the temp starts to climb, and then the modulation can throttle the boiler back, to ensure a gentle arrival at the set-point. If the boiler had been range-rated at say 70%, then even with or without modulation, the property would take much longer to heat-up.
In practice though, I guess one should at lest set the heating to at least a level to protect against frost; but even then, the max output will help. And yes, I'm getting a fancy Wiser control kit, so I can turn on the heating at home when I take off from the Bahamas!
As it's now December 1st, I wish you all a very Merry Christmas, and a Happy New Year. 🎄🎅