Thanks everyone, this has really helped. After thinking about what types of photos I mainly take ( landscapes/seascapes ), I think I'm going to sacrifice the long zoom over quality. So given I've only got £600, what are your thoughts ?
So here's the issue, assuming similar age tech then the larger the sensor the better the image quality but then the bigger the lenses need to be to cover that sensor. This is why you can get a compact 24-1200mm lens in a bridge camera as it only has a 1/2.3" sensor, compared to a FF sensor which is roughly 36mm x 24mm
The next thing is the lens, lens quality has a big impact on image quality. Faster/brighter lenses with wide apertures tend to be heavier and larger. So as you will be becoming aware of, it's always a compromise between size/weight, image quality and cost. I've been searching for the holy grail in getting the best IQ but in a small lightweight package and I am still looking. The best compromise I found was the m4/3 system and am a big fan of Olympus. Image quality is excellent and size is manageable', however depending on the body and lens(es) you choose it can still be considerably bigger and heavier than your bridge camera.
The first 'step' into getting noticeably better image quality (IMO) are the 1" type cameras, such as the Sony RX100 series, the Canon G7x and G5x series, and Panny TZ series (some of them). Sony and Canon have the advantage in that they have faster lenses allowing more light to enter the camera and therefore will perform better when light is less than desirable. The TZ's have the advantage of extra reach (zoom) but at the sacrifice of light gathering, at the long end less than a quarter of the light that the Sony and Canon allow. One consideration with the compact cameras is that whilst they're good for carrying around the size means that they 'can' be fiddly to use.
The next step up is the Panasonic LX100 and LX100 II which use an m4/3 sensor although it's slightly cropped and so not quite the full size of the sensor. These again have nice bright lenses and offer better IQ than the 1" cameras, but at the expense of them being bigger and having a 'limited' zoom.
Next step (in terms of sensor size) is the m4/3 system which can produce excellent images. You can have a pretty small setup like the EM10 and pancake zoom which will take very nice images, but light gathering of this lens isn't the best. You can get a superb fast (that means it gathers a lot of light) zoom lens in the 12-40mm f2.8 but it's larger and more expensive.
There are then fixed lens APS-C cameras which have bigger sensors still. Some have zooms such as the G1x series but at the sacrifice of lower light gathering, and then you have fixed single focal length cameras such as the Fuji X100 series and Ricoh GR series.
You then have the interchangeable DX and FF cameras but we're generally starting to get into bigger, heavier and more expensive gear here.
Here's a comparison of sensor size, what you have currently is the 1/2.3" (bottom row second from right)
camera-sensor-size-12 by
TDG-77, on Flickr
And here's a comparison of the size of some of the cameras I've just mentioned.
Screenshot 2020-06-05 at 13.16.30 by
TDG-77, on Flickr
For a lot of people a 24-70mm is a pretty good allrounder lens.
