It's very hard to tell. There are quite stringent regulations about where you can site boilers, in relation to venting, how close they can be to opening windows, etc. It is possible to turn a £2k standard job into a £5k pain if a lot of work has to be done to site the boiler. Also, design, quality and rating of boilers vary a fair bit, though a small flat should only need a small boiler.
That above price comparison (the £2k and £5k) was, by the way, for two identical houses next door to each other, installing identical boilers. One was replacing a boiler about 5 years old where the boiler location met current regs, because it had previously been moved, and the other was where floors had to be lifted, gas and water pipes rerouted, external flues installed, electrical work rerouted, etc, because it hadn't been replaced in 25 years and the old location no longer met gas regs, so the new boiler couldn't be put where the old one was.
I'm not suggesting yours will be either £2k or £5k, and they were both in 4-bed houses. My point is that two superficially identical jobs in identical houses, with the work done by two crews on the same days, were quoted that differently, by the same heating firm, because the one needed several days more work than the other. Which makes it very hard to put a price on a job. Oh, and neither of those jobs required either radiators or pipework for rads, as that was already there.
Also, how you spec it will make a difference. For instance, radio-linked thermostats will be a bit pricier than cabled units, but the extea is very likely a lot less than the labour to run cables from room thermostat or tank (if any) to the boiler controller, especially if you have to start chasing out walls to hide the cable. Just go radio, IMHO. Or "smart" if that floats your boat.
