I replaced mine with a Worcester Bosch this year (a Greenstar 30i) for a 2-floor plus cellar & attic terrace (just one bathroom, but boiler-heated shower). It was similarly a direct replacement for an old combi (a Potterton).
Current regulations mean that all new boilers have to be either load or weather compensating. In effect this means that everyone is bundling a "free" smart thermometer. If you need more than one zone, it may be worth getting something more complex - the Bosch one works with electronic TRVs on all the radiators ... but only works well if you have TRVs on all radiators and is more expensive. For me, a single thermometer/zone worked fine.
I bought the boiler through boxt.co.uk. I think I would have had better service if I had used a local installer which had been recommended to me, but I can't fault their speed - I ordered it, and it was delivered and installed the next day.
I would advise you to use the boxt site though, even if you're not going to order from there - it will ask enough questions to size the boiler and work out which accessories are needed (e.g. flue extensions). It also gives you a fixed price for installation, which can be used to benchmark other installers, if you're not using boxt.
Apart from the installation, which wasn't perfect - long story, the installer took photos of the installation and was sent back to do remedial work by Boxt/Worcester Bosch before they approved the work, so they definitely looked at the photos.
I'm entirely happy with the boiler. The Honeywell thermostat works pretty well, and I can control it from my phone remotely, which is nice. I worked out it'll save a fair amount, too. I'll post the graph if I can find it.
I didn't spend a long time looking at boiler makes, but I did know that I didn't want a Vaillant, and I had heard good things about Worcester Bosch. I picked the size as it was slightly bigger than my old boiler (in terms of capacity) but smaller in physical size, so wouldn't have a problem with fitting.
Be aware that if you have a 15mm gas pipe, then this may need to be upgraded to a 22mm pipe. I didn't need to in the end as they tested the pressure/flow, but it would have been included in the cost if needed.
I hope this helps!
Edit: here's the graph of my gas usage. The weather was getting a little warmer, but I do think it'll save me money long-term:
