If you don't have any issues with your current setup then it probably isn't worth changing to a combi. There's extra work involved rerouting pipework and possibly gas too, and this feeds into the extra cost. If you really want to get rid of the tank and are aware of the pros and cons of combi boilers then go for it. To do a proper job of switching from a system boiler to a combi boiler, your quotes don't sounds too bad. There's a big difference depending on what part of the country you're in. I would have extepected a like for like replacement to be quite a bit cheaper, but then they might be quoting for a lot of upgrades for like a fully pumped system, TRVs, etc.
Only downside I can see to combi is lower water pressure.
Not really. Combis provide excellent water pressure, as high as you want really. The downside is that they can only heat the water so fast and so can only provide hot water at a certain flow rate. Increase the flow rate too much, for example with a power shower going full blast, and the temperature will drop because the boiler simply isn't powerful enough to instantly heat 20l/min to 40C. Get a bigger boiler and it will be able to heat a faster flow rate to the temperature you need. In practice, most combis that people put in will provide for a single (non-power) shower with nobody else running hot water at the same time, or fill a bath in a few minutes. They can also take slightly longer for hot water to start coming out of the tap, but in practice with modern boilers and warm-start functions this is mostly controlled by the length of pipe from the boiler or tank. Because combi boilers are nearly always more powerful than you would need just for space heating and gradually refilling a tank, and of course because combis are considerably more complicated, that can add cost.
Thus plus side of combis is that you are only heating the water that you actually need to use. This can be a big advantage for some people, but perhaps not for a family of four. Maintaining a tank of hot water inevitably means some heat lost but it can often be useful for keeping an airing cupboard warm, and if it is inside a house the "lost" heat is still useful at most times of the year. Another plus is that you never run out of hot water when the tank is empty. You can literally run hot water forever, although the water will only come out at typically 10l-15l per minute. Handy if you like taking massively full baths, or if a whole series of people like to take showers one after the other, but you should already know if this is a problem for you. A larger tank is always possible.
they require a really clean heating system
Huh? This is hardly an issue specific to combi boilers. You may be assuming that recent trends towards paper-thin heat exchangers and issues with touchy condensing boilers somehow just apply to combi boilers, but combis have successfully been running sludged up systems for decades.
