Unrelated to the problem being addressed: I notice that your speaker wiring appears to be reversed polarity......check the back of your Cambridge to see if that black wire is going to the positive. If so, then it's just the colors that are not matching the polarity, i.e. red usually goes to positive terminals and black to negative when used together in a pair. Check your other channel as well.
Now, unplug your turntable RCA outputs from the phono preamp and plug them directly into that input on the rear of the Cambridge that is marked MP3/Aux.
Does the sound change? Remain the same? If it remains the same, your phono preamp is likely at fault as it is not applying the increased volume (gain) and something called the RIAA curve which is needed for things to sound as they should. If it changes tell us how......the sound should be very weak and tinny as you have described in your first post. This is just a check to see if the phono preamp is doing what it should or not.
Beyond that, you've likely already tried a different set of RCA cables from your phonostage to your Cambridge, right? If not, do so to see if that changes things. Not likely because if there is a problem with these, it's not as common for it to be with both channels, usually just one but I have seen it happen that both are bad.
.....and, I think you've already tried it, but plug the RCAs from the phonostage into the CD input on the Cambridge and tell us what changes.
Now, you've covered the gamut of likely problems/easy fixes and if all remains the same, time to think about the cartridge end as others have mentioned, the wires there and even the RCAs coming out of the turntable.
It can be frustrating when things don't work right, but rest assured.....there is an explanation for what is going on and you will find it as you keep digging. Then you will be able to enjoy your setup as intended! Keep us posted amigo......
PS - When examining the cartridge end of things, I would start by looking at the colors of the wires and making sure they are going to the right posts on the rear of your cartridge. It would appear that, when looking at the rear of the cartridge, the RED wire should be going to the top right post, the WHITE wire to the top left post, the GREEN wire to the bottom right post and the BLUE wire to the bottom left post. Make sure they fit snugly and if not, adjust accordingly.
Also, get rid of any ground wires you have attached for the time being except the one from the turntable to the phonostage, assuming that one is a Rega wire that is part of the wires coming out of the turntable. That is not likely related to the problem, although if the cartridge wires aren't attached properly it might be causing issues on top of issues which are BIGGER issues.....
