One oft overlooked point is that domestic natural gas supplies to domestic houses is typically 66kW maximum unless you have a commercial supply instead, a bit like the electricity board's 100A 1 phase supply or 3 X 100A 3 phase supply.

Why is this important? Well my house has a 38kW vaillant combi. So that leaves me a 28kW supply margin for my two gas ovens and 6 ring hob.

It is possible to build combis with higher rated flat plate domestic hot water heat exchangers and burners. However you run the risk that trying to consume gas at a faster flow rate than the gas meter and governed can supply, the internal gas pipework pressure will drop below 22 mBar.

Supply Vs demand issues for LPG or oil tanks may not be a factor as presumably they can be designed to be able to supply what the peak demand of the house is going to be, i.e. the boiler going at full pelt if rated higher than a domestic gas boiler and with the cooker going too in the case of LPG.

I am sure an OFTEC or LPG accredited engineer can offer more comment.

So to the original op, you may find that a decently sized combi on a decently sized LPG/Oil tank may not have the same issue as encountered by natural gas combis.