Good answers which I can only echo.
Here's another important angle - the cost of the new boiler is perhaps about one-third of the total cost of changing the system around - the rest is labour (I'm assuming the flue arrangements have to be altered for a fan-flue, and a combi system is replacing an older style domestic HW tank). It's winter and good plumbers/heating technicians are busy anyway. The Government has just upped the demand versus supply, by introducing the change for G-rated boilers (yet another barking-mad idea, announcing it at this time of year - still gotta try and win the election somehow, eh?).
So, sure think about, but don't DO anything until about 1st June. Otherwise you will just pay through the nose on the labour cost.